All you have to do to get a pre-board pass is ask. MY parents both have mobility issues (mom has RA, dad has a bad back that affects his legs and makes walking difficult) but neither of them use assistance devices, so it's not obvious. All they have to do is ask for pre-boarding and they give them the slip.I'm not certain but the OP may have a different situation then wheelchair users. From thier decription of the problem I'm thinking that they may have a disability that is not mobility involved. I'm not sure how SWA would handle that.
Sometimes they ask why they need them, but it's not a problem.
I'm sure if you explain your situation, there definitely won't be a problem!