Yeah, that works good. Make the Stereo really wide, and if there are like basses/tenors/sopranos try to 'group' them so you can have a more 'choir-like' sound. Usually the bass and sopranos goes on the top ends (like the lower voice 100% left the higher 100% right), but you never know.. i've heard some weird choirs arrangements for great effects.
Compression on the whole does good (just don't add toooo much), and a good ambience is essential for the boom.
Good harmonies are a great deal too. Try to wide the range of the 'choir', by adding some low bass arrangements to high tenor vibratos. This makes the whole deal grow.
Sorry 'bout the large post

. Backing vocals aren't a great thread in a metal forum, unfortunately. See ya!