I would not recommend EVER muzzling a dog with seperation anxiety. Especially if he's drooling and panting excessively. D: Muzzles are very stressful, even if you desensitize him to wearing a muzzle. I would never leave a muzzle on a dog and leave them alone.

It sounds like an herbal supplement, like Rescue Remedy, could help him a lot. I gave Fozzie RR when I first started working on his SA. It helped a lot, and he doesn't get nearly as worked up as it sounds like poor Springen does.