The ReplayTV 5000 series works easily with the Comcast DTA-- with two caveats.
To use a Replay TV 5000 series with a Comcast DTA you need to:
1. Get a Motorola-branded DTA from Comcast. The model number is model DTA100/2305/000
2. Obtain on eBay a "Tivo to Comcast DTA" cable from Gary Gray. He markets it as a TiVo cable but it works perfectly with a Replay TV-- at least the 5000 series. His eBay user name is g-gray. Just search for him and you'll find the cable. Make sure you get the "TiVo to Comcast DTA Cable", not his other cable which costs a few dollars more.
3. With this setup, the Replay TV works fine in concert with the Comcast/Motorola Digital Transport Adapter, using the default Motorola IR code-- no software modification is necessary for the Replay TV.
I have not tested this with any other Replay except the 5000 series, but the world is curious if owners of other models would try it. I believe Gary will refund your money if his cable doesn't work for you.
Long Live Replay TV!