re: adding user defined streaming video sources to set-yop box like vunow
when: i dont know when it will be rolled out for the vunow, but i am confident it is intended to happen
where: eventually on the vunow; as far as other set-top-boxes... i don't know of any with this as an 'advertised' feature. i would expect that it is fairly obvious to all vendors that this could be a useful add-on and at this point adding it would make their product uniquely more desirable.
get your fix now: there are other ways to get your fix now, albeit fairly convoluted ways.
for example, i can add arbitrary video streams to Video Center on my Nokia N95 mobile phone, and via the TV-out feature i can watch those streams on my TV at SD resolution (640x480). sure, that's almost like bringing a laptop to the tv, but in this case it's a smaller device i already have in my pocket and connects natively to the RCA inputs on my TV. cost? yeah, the N95 is 2 years old and it still costs over $500 new.
so, unless you already have one, the N95 likely isn't a valid alternative to get-your-fix, but you may have something similar to do this already. maybe you've got a desktop that you can setup to relay the stream(s) of interest via UPNP (search for Digital Living Network Alliance; http://www.dlna.org/home
-- this is universal plug and play and this is what vunow already supports and i use to stream mp3s, video, and photos from my N95 to my vunow). The vunow will connect to and playback pics, vids and tunes from DLNA compliant UPNP devices. they only test with and provide directions for talking to windows media player in this manner, but if you loo at those directions you'll see that they refer to setting up windows media player -- the vunow requires no setup for this task, any UPNP device on the network just shows up and works. my N95 just shows up and works, and it has nothing to do with windows media player (does NOT run windows of any kind). maybe that will work for you? it's a hack-a-thon to get this sort of feature when it's not officially provided for you, and sadly that's the only direction i can steer you for now.
otherwise, you've got to wait until someone at verismo, roku, or vudu says that they're shipping the feature - and i haven't seen any public information that this is coming soon on any of these platforms. (i'll refer to the content of my related blog post as non-public information; it's not from the website or an official verismo press release).
sorry i can't help more.