Great little fantasy. Could maybe make it work with the right gear....

Standard balloons have the issue of introducing "raw" latex to sensitive tissues. Even for people without a (current) latex allerergy, latex is one of those substances that can hit you hard the moment the allergy kicks in...

I'd look for something along the lines of an extra heavy duty condom....

Never introduce air. Pressurized orally, it will be loaded with germs that would infuse into the tissues if the balloon breaks. Pressurized mechanically, you risk introducing machine oil...

Water, previously boiled, introduced via something like a sterilized turkey baster... yeah, that could be the ticket.

Hot or cold? The obvious temperature advisories should apply.