It sounds like the URC command doesn't repeat but the Apple TV command lets the command repeat as long as you hold down the button. I've seen something like this way back in original Pronto days!

At that time the solution was to envision the command as having two parts. The first part was learned the normal way. The second part was learned by covering the remote's emitter, pushing and holding the button, then uncovering the emitter. This learned the repeating part of the code. Then you'd make a macro of the first part followed by as many as a dozen of the repeating part. This was how you'd construct a projector power command back then, where you had to hold down the original remote's power button.

The thing is, this worked with the first couple of generations of Pronto and Marantz remote. Once the software changed to NG, the repeating section apparently had pauses in it.

It might be worth trying to make a macro like that.