Thank you for your replies everybody!!!

I will be trying a new technique tomorrow. *crosses fingers*

I received a check-up phone call from the clinic today, and I was able to ask the nurse for some more suggestions. Unfortunately she told me that what I had been doing was what she'd have suggested, so tomorrow I'm going to get a phone call from the vet herself.

With any luck, by this time tomorrow he will be opening up like a champ!

I'm also going to ask about injectable meds instead of oral. I'd rather not have to give them orally if possible. I don't like stressing him out by touching his head, even though he seems to get over his head-shyness rather quickly with me. This morning he stayed quite still and let me 'play' with his lips, even though he didn't open up.

Salzedo is my only snake, so I don't have any others to take back to the clinic for a hands-on demonstration. I feel so awful about this. It really was a freak accident. I observe all feedings, and Salzedo's response is fantastic; the mouse is gone within 2-5 seconds of being introduced to the feeding tub. Its just that this time he coiled around the mouse in a way that I could not see where its head was, and by the time I located it, the damage was done and it was dead. At least he ate the damn thing.

Thank you again for your suggestions and thoughtful messages. It really means a lot. I'll update this after I speak to the vet tomorrow and attempt another oral administration, providing I don't get injectable meds instead.