GHS has sets of
Boomers (.011-.053?) and
Nickel Rockers (.011-.058?) that are geared towards low tuning... in fact, they're called "Lo-Tune" sets.
The reason i put question marks on the gauges is because there's a conflict between what juststrings.com says they are and what GHS says they are... amazingly, i think
GHS' site is incorrect, since the gauges they list for the Lo-Tune Boomers are exactly the same as their thin-thick set (.010-.052).
I'm probably going to try the Boomers on a guitar i'm building specifically for drop-tuning. If they're a bit too loose, i'll try the Nickel Rockers, since they're made from a harder alloy and apparently have a stiffer feel.