If riscos is in millions of set top boxes worldwide then it is capable of controlling a device made for delivering video media to a TV screen (Yes it can not do it through a software front end on current desktop hardware but in a Home theatre PC you would want the video full screen anyway so this could be delivered the same way as all the Risc OS STB's did it - esspecially if it is a STB-esk computer).
Also Castle has said on a number of occasions that I can remember they did a lot of work on media replay with really good results unfortunatly due to it being comercial work the are currently not free to release anything from it (though I got the impression it seemed to be that they would be able to use it at some point).
There is also a superb free RDP client that runs very nicely on just an SA risc PC with a 10mbit connection (infact it runs very nicely indeed) So any modern Risc OS harware with 100mbit would easilly do very well indeed at allowing access to a desktop machine from TV.
Again RPC can handle music replay fine whether it be CD or MP3 so music delivery is ready to go.
Also in the respect I see the hardware being sold multi-tasking isn't as big a deal as it is a media player first and you are single tasking while watching a film (playing music may benefit from being able to select a track while playing one so some basic multitasking) - It would need a nice media browsing front end.
I am not proposing it be a desktop replacement... just a route back into new customers homes but with an up-to-date cpu it would be nice for current Risc OS users as well. RDP makes a unique selling point (a lot of people have PC's in studies or bedrooms).
Then they discover it has it's own desktop with some built in software...
If it's cheap enough they buy the kids a media player for their room... They have a play with it and get used to the desktop and start using it for computer tasks....
I can see this being possible as media player / stb hardware is manufactured in bulk by OEM's and the pricing could be reasonablly keen compared to harware built bespoke for risc os or using an off the shelf eval board.