Actually, I've talked with Ruka about this, and I've been meaning to archive the works by other groups in the wiki for a while now, I was just procrastinating on it. There is indeed the concern of hotlinking without permission, so I was intending to contact the groups first (and maybe see if they can do the archiving themselves).
The notion of the wiki as exclusively for Gaku releases probably arrived because at first Gaku was the only group doing Touhou scanlations, and also because the wiki was originally hosted on pooshlmer (and has kept pretty much the same structure through the host changes), and at that time the scanlation work was also done at poosh (I know it sounds weird, but it was a different time, you had to be there). Later, even when WoY and UMAD started doing a lot of releases, I guess the notion stuck mostly out of inertia, which is why non-Voile additions are few (but not non-existent, the SaBND page links to both Gaku and UMAD versions of releases, for example).
As for the question of dead links, the vast majority of Gaku releases are not actively hosted in the gensokyo.org server anymore, but wiki users took upon themselves to provide alternate MF links, so I believe this would work just the same for other groups' releases.
I don't know what is the position of the wiki admins on this, but as far I'm concerned if it helps the Touhou fanbase then it's alright.
