I just received a deprecated TV set-top box (Telekom Magenta TV MR401B) from a neighbor and it seems to be a decent piece of hardware with a 500 GB disk and embedded Linux as far as rumors go. It is based on a Broadcom BCM7252S SoC.
What would be the first steps to try and support that as a platform for OpenWRT?
Obviously, it’s not a network/router platform but it seems “real” NAS distributions are quite resource hungry. (Plus, I am used to OpenWRT).
This is pretty much a dealbreaker right here. Broadcom devices are (in general) very poorly supported -- especially the wifi side of things. Very few people will be interested in assisting here because Broadcom doesn't open-source their code/drivers so it means reverse engineering and developing from scratch. There is support for the 27xx, 47xx, 4908, and 53xx family devices, but you'd probably need to begin fresh for the 7252S.
Even if you would get drivers for the more complex parts of it (all the graphics and video stuff) (it's Broadcom, forget it, you won't even get wireless-, modem- or phone drivers from them), OpenWrt wouldn't exactly be well-suited to do video playback or do anything more complex than text on the tty.