Well, took another look at some of the components here.

Installed UPnP on the N900 using Rygel. The music collection turned up in both X@Windows XPTE and on the N810. Plays in the default media player. So, the N8x0 working as a stationary UPnP player with the N900 being the UPnP server - check.

Now I've started looking into hibernation variations, currently browsing many theads about subjects like suspend to drive. There has been some activity around it on this forum [as some previously active in this thread probably would remember from having been a part of it] and elsewhere (for ARM).

Even if I get suspend to drive/hibernate to work, it'll still require a trick or two before the device hibernates and wakes on charger power off/on. But with the pointers from qwerty12 over there, I believe it's quite possible.

Honestly, I believe qwerty12 can get more out of a Maemo device than Nokia ever envisioned. They should hire him. :B