My thought is using a WB-like holder on the toptube with extra elastic or toelip straps for security. You could have a simple clip on the stem to keep the hose in place. Then maybe a loose, elastic necklace so if you drop the mouthpiece there is no chance of it getting caught up in the bike running gear.

Edit: this could work IF the 3 1/2" bottle will fit between your pedaling legs. For some, no issue at all. It wouldn't work for knoch-knee'd me.

And another chapeau! Good for you!

Ben