But to clarify your question - you're saying that if the sealant does work, but you've lost too much air pressure to continue safely, how do you inflate back up? I'd say don't do that. If the pressure goes too low, just replace the tube with a butyl tube, re-inflate, and be on your way with high confidence. The goal of sealant in tubes would be to have some chance of sealing small punctures very quickly, so you don't have to stop.
As soon as you have to get off the bike, just fix the flat properly. If the puncture was so bad that it didn't seal until you went nearly flat your odds of successfully getting back to higher pressure with a tubed setup are pretty low, and you're better off just committing a few minutes to a full tube swap.