The trouble for me in this sense is that I have an AVR, I'd like CEC to work and I'd like an interface that was actually designed for a remote.

I've done this before, and frankly it's just not as good. It works reasonably well, but an HTPC has never served as well as a purpose built piece of hardware or software, even if that's the TV itself.

If I really wanted, I could buy a hospitality TV and use the RS232 to control it from the PC but I'm tired thinking about the ways that's likely to fail.