I do not believe in inflating massive DSMB's from depth because they're a PITA to deal with as you've realized. I don't dive with a backup DSMB, I dive with a small 3ft skinny thing that I shoot from depth, and a big **** off monstrous thing that I only inflate at the surface as a "come find me, I'm lost". For that one, you have time to deal with it. Otherwise with the huge one, just inflate it from as deep as you can and you don't have to have that much gas in it with expansion.
This is exactly what I do as well. I have a small Halcyon DSMB (3' skinny one) that I shoot from depth. I have a small BC hose on my pony/stage that I use or I orally inflate it. Both work just fine especially if done from 50-60' or deeper. I then have one of the BIG Halcyon SMBs as well that I can inflate at the surface if necessary. I "could" inflate the big guy at depth if need be. I keep a spool attached to it and ready just in case. But I've never had to use it from depth. I keep one DSMB in each pocket of my pocket shorts.