As an owner of 2 SDTivos and 1HDTivo, I'm suprised the SD Tivos can't be configured to control a $40 OTA Digital converter box that has IR remote. I would search around a little more on this if I were you.
The Xtender video quality is quite good, but only as good as the input signal quality up to 480 lines (720x480) of resolution. The Composite Video output from my HDTiVo is noticeably better than that from my SDTiVOs. I think the HDTiVo channel looks very good on all of my TVs including my 52" LCD, and acceptable on my 110" projection screen. The SD Tivo's look a bit softer. Seating distance from the screens is a factor also. My wife is completely oblivious to the differences so it depends on your sensitivity to video quality differences too.
Yes, The HDTivo works great with the BOCs Xtender Hub! And yes, it looks even better with HD source material than SD source material!
I don't know if there is any kind of integrated Macrovision type protection on the TiVO's, but it doesn't impact the BOCs Xtender Hub's functionality, so I'm guessing not. I know there is a VCR recording function on the SDTiVos for backing up recordings to VCRs, but I never tried it. As far as copyright protection, I think since you are going to the analog domain, it is OK to record the programs for personnal viewing purposes, but am not a lawyer.
Good questions on the Harmony remotes. To use in a room other than where the BOCs Xtender Hub is, you need a compatible RF path to the Xtender. If your Logitech remotes are the RF ones, I think you could put the R receiver next to the BOCs Xtender Hub and use the IR Blaster wire from that attached to the IR receiver on the BOCs Xtender, but you would need to verify with BOCs as I have not tried this. Maybe an IR remote would work in the same room as the BOCs Xtender Hub for the same reason, but you would need to verify.
I don't have a Sling Box, but conceptually this should work, but again, I would ask BOCs customer service as they probably have customers who are doing this if it is possible. Go to www.bocsco.com
When distributing audio, there are many ways to do this depending on your setup and end goals. 1) Use a source, like HDTivo, with either Rhapsody or linked to a computer with your MP3 collection (Tivo Server), etc and just control it like you would today via the HDTiVo interface and through the BOCs Xtender Hub. You can serve your photos this way too. 2) Use a more direct source like an IPOD Dock or a computer with composite video output that can be controlled by an IR Remote and set it up as a source for the Xtender. This is going to be one of my next projects after add 3 more sources through an additional BOCs Xtender Hub!
No you won't be able to hook up a FM receiver directly to the COAX and get FM Radio on all your TVs. Think of that FM band over coax like an antenna input to your FM receiver from the cable company.
You could run the analog Left Right Audio Line Outs from your FM Receiver as an input to your BOCs and then the associated channel would have that sound play through the TVs speakers, but the remote control would be a challenge unless your FM receiver has an On Screen Display and comosite video output for tuning channels, etc. Hmmm, I need to check my Denon to see if it supports this!
Hope this helps a little!