Thanks guys, I really appreciate your ideas! It seems that this is not the sort of thing that will have a strong consensus... It really all seems to boil down to personal taste.
Personally I'm drawn to keeping things as simple as possible, to avoid a very messy database as Mika suggests!
I tend to agree with tagging the singer as the artist, since this is consistent with tagging the singer or band of an album in the artist field (All my music, both vgm soundtrack and artist album are all together in one database, so I don't want to have one rule for soundtracks and another for albums). So with the Silent Hill soundtracks I will tag Mary Elizabeth McGlynn as the artist for the applicable songs. Akira Yamaoka will still show up in the composer field, but I keep that field hidden for simplicity and aesthetic
I also think it seems more appropriate to tag the arranger as the artist. Again, for me it is about keeping things simple. When I am looking at my music in iTunes, I would like to easily see who made the music sound exactly the way it does (without getting too technical), not who wrote the very first (and different sounding) version of that song. Simply put, I feel the arranger is very much like a performing artist (singer) but without the words. So if I am going to tag a singer in the artist field (and not the composer), I should also tag the arranger (and not the composer), otherwise I would be inconsistent.
So that's how I've decided to go about my database- By tagging the arranger (where applicable) as the artist instead of the composer, and by tagging the singer as the artist again instead of the composer
This is the simplest, and most consistent way I feel it can be done, without creating a complex and messy database full of ,'s and /'s