Yes, you can use RTI to send commands to a HAI panel, and yes I have done it.

HAI has two RS232 protocols: Omni-Link and Pro-Link. I don't think you can use Omni-Link with RTI because it requires an authentication process that is two way. Omni-Link works great with Crestron (I done that too), and Tony Golden has some modules for Crestron to HAI integration that work great.

Anyway, on the HAI side:
1) Pick the serial port you'll use and set it to "Pro-Link". Set the baud rate to 9600 (the default). This is under the setup-installer-expansion section.
2) Create button(s) that will do whatever functions you want when activated.
3) Create message(s), basically short ascii labels that represent the functions you will activate via button(s).
4) Create program line(s) that activate button(s) based on the receiving the message(s) you defined previously.

On the RTI side, you'll need a RP6 and a CM232 module. Plus you'll have to either make a cable or buy the HAI serial to PC cable to connect the two. (The CM232 terminates in a DB15M, and the HAI serial ports are RJ11s unless you buy the optional HAI RS232 add-on board which has a DB15F connector.)
Then:
1) Create a button on a RTI page and associate a RS232 command that is the same ascii string you defined above for each HAI message. For example, if the HAI message was " Lights ", then you would have a string in the RS232 command that is " Lights\r " which sends the string and a carriage return. The string is entered without the "" marks.
2) Set the serial parameters for your RS232 command to 9600/8/None/1 .
3) Set the output of your RTI button to the specific port on the RP-6 where your CM232 module is connected.

Using this method, you can kick off any HAI button (macro) from RTI. A macro can obviously contain a single command or several. For example I've created a "Good Morning" button that sets thermostats, turns off the alarm and turns on lights.