On my cheap, bare bones set up, I have a on camera flash (do you still have the 580? and the 85? that I either bounce or use bare strait at them, depending on what I have to work with.
100mmL macro is great for both ring shots and portraits, decent in low light. My worry is where you only have florescent light.. hopefully you can bounce there with the flash.
So that is my suggestion, if you have the 580, check out this site, and practice your butt off between now and then
This guy has the best bounce site instructions on the net.
Neil black foam thingy
Rent a 100L, and then borrow/rent another back up body.