Fabric = insulator......(clothing) won't be accurate

The "Disk" sensors that I have seen look like they are flush mounted but they are not. There is a small, mandatory gap between the back of the disk and the wall surface. This allows air circulation. Painting these sensors is not a big deal, but one should obviously avoid a dark color that is in direct sunlight or strong lighting.

I think that you should not even attempt to place the sensor behind fabric. The very best outcome will be that system response will lag the room air temperature. Room air temperature is strongly tied to comfort, but wall temperature is part of the comfort factor too. Sure, you could argue that these are internal rooms and there will not be much of a gradient through the wall, but if the load changes, such as adding party bodies or turning on some lights, comfort will suffer. Everyone will be a sweaty mess before the system responds and after the party, the lollypops will freeze.

Mounting and evaluating the test panel will be difficult and time consuming because you'll need to change the room load and log the results. This would require a fully functional HVAC system and this is probably not available at this point in the project. A proper test will require data logging over a period of days and a convincing presentation of the results to a technology blind, not so receptive audience.

Mounting the sensors higher in the room than recommended is not the end of the world, but there will be a temperature offset.

If the designer is going to have a tantrum about the sensors, and this is likely, have the designer sign off on the HVAC plan and have the designer be responsible for hiring a consultant to come in later and fix things. And everyone should be aware about how expensive replacing a sensor behind the fabric will be.

Another tact would be to hire the consultant now to give an opinion. This consultant should have credentials and not be associated with any current stake holders. (architect, designer, contractor, or home automation guy) The consultant will have more experience with presentations.

On one of my sites (A/V, no automation) the thermostats were mounted a few inches above dimming light switches. I pointed out the folly of this to the designer and contractor before the walls were painted, but by that point everyone was fed up with the designer's follies and the thermostats were not moved.

I don't recall ever seeing this offered as a home HVAC solution, but an infrared sensor could be a great solution. The IR sensor can measure surface temperature without being part of the surface. The trick would be to measure a surface with a relatively low thermal mass that tracks room air temperature.

I'm not an HVAC expert, but if this is a constant air circulation system (always circulating), sensors in the return air duct might be practical, but there will be seasonal adjustments with regard to the temperature offset.

Have the designer think about things. Really, if the thermostats were not important, they could all be hidden in a corner of the basement. And, why would you need more than one?

I'm not condemning all designers, but some of them need a physical reality check. For these individuals "art" trumps technology and the physics can be changed as easily as, oops, wrong color. All they are worried about is the final photo. (and payment)
 

Designers should be forced to actually live in something they have designed for a year....

....so many times I get to see design succeed aesthetically, only to miserably fail practically.....

They pick up their cheque as it all looks good.....and then three months in, the technology guys (and I'm including plumbing, HVAC, electrical, etc. here) get blamed because things don't work as they should, more often than not despite the numerous objections they voiced at the time (paper/email trails with disclaimers/professional advice are normally your friend here).

As previously stated, get an independent consultant involved that will provide a Go/No-Go statement and then get the designer to acknowledge it.

I would pursue a return mounted sensor option based on what you have told us.

Since you have radiant controlling the base temperature, the temp just inside the return grill should closely track the ambient air temperature (assuming your HVAC designer has properly located the returns).

Doing this will allow you to place the sensor in a free air environment and also not worry about the aesthetics of the sensor.

Since the sensor is in a moving air corridor, you'll want to make the thermostat react more slowly so you're not short cycling anything, but this should be doable.

Return air sensors are relatively common in the commercial HVAC world, if nothing else, seek out a friendly learning conversation and see what they think.

Brad

I have to side with the designer on this... if it was just a disc sensor it wouldn't be that big of a deal to mount through the fabric and paint, but the Honeywell sensor is a plastic box that's 1.5" wide x 2.25" tall by 3/4" deep. I wouldn't even want them stuck on the walls of my house.

Return sensor might makes sense... I think the system might have constant airflow.  The HVAC system is all commercial in design and is being installed by two different contractors.  One is doing the ducting and heat exchangers and the other is doing the boilers, chillers and radiant.

The boiler guys are world class with tons of experience and can probably provide some guidance so I'll setup a meeting with designer and both HVAC contractors.

Here's photos of two of the boilers and one of the chillers.  Not your average residential stuff.