Use a stool - you may get some stick for it but so what? I use one whenever I can, and after a few years of getting dodgy looks from bandmates I've noticed a few guys following suit. If it's a gig that it might look out of place sitting down, get a high stool, with a high foot rest, and semi sit/stand - ie: with your left foot on the ground and your right foot on the foot rest, giving support to your bass. It might help to explain what's going on to the band leader or anyone else that might have a problem with it.
I do sympathise - I did a few gigs with a broken collarbone years ago (thankfully on my right side, not the strap side) but I still needed help getting the bass over my head. Get some rest!!!!