Here's a thought:
While the guitars have tuned down, based on my calculations, if you tune the 5 string up a step it will go from B E A D G to Db Gb B E A
That way you'll get an open Db (I) and an open Gb (IV) to work with and the I-IV-V within easy reach on the two lowest strings. And you can play the 5 string with the same patterns/boxes/finger spacing as you do with the standard tuning.
Might have to adjust the truss rod to compensate for the additional tension.
Something to think about. Correct me if I'm wrong.
