I really like the way you are going with it and have 2 questions that come to mind.
First is the same that I asked in the power saving topic I started and similar to possy's remark: is it possible to watch live mythtv that way (and control recordings etc, I have my tuners in my core).
Secondly I personally try to avoid wifi and have everything wired. Are there cheap solutions to have a fixed orbiter and are there remote like controllers (I currently have a gyration and a mele F10) that can still be used?
Well if you want to watch Live TV then you can still watch it via your TV itself using its tuner or an external feed from an STB etc. For VDR/Myth then no you cant watch Live TV streamed directly from either of those servers. However with Myth & VDR (I think its tru for VDR) you can watch your recorded shows via upnp and a media player or SmartTV.
As to wired v wifi - we've found that if you provide enough wifi coverage (ie multiple AP's located throughout your home to make sure coverage is good) then wifi performs well. I would say 95% or more of our installations/users in the last 2-3 years are using wifi because they use smartphones/tablets for their Orbiters.
You could use your Gyration/F10 but that would mean using MD's as you need a traditional TV Orbiter UI for those to work or make sense. Its your choice in the end.
When we ship our new HTML5 UI then that will not be available on an MD - or at least we wont be offering that. But anyone who wants to could develop an MD with it integrated if they wanted to ;-). The other possibility is to access the HTML5 Orbiter from a SmartTV using its Web browser...we've not tested that config yet...but as our Orbiter is just HTML5 there is no reason why that would not work at all ;-)
All the best