Actually, another idea that seems solid- first, you grab a shank, since it'll do d4 damage and you can hide it from the guard. Get the guard to come into your cell- they don't need to think you're sick or anything, you just need them in there- then I'll cast Heartache from BoVD on them. Heartache makes them helpless for one round, which would let you do a coup de grace attack with the shiv. Assuming you have strength ten and don't bother casting magic weapon, that means that they'll have to make a DC 18 save or they die. I'd lure him into my cell to do it since I have more strength and the save DC against death would be higher, but I can't cast Heartache and spend a full-round action doing a coup-de-grace all at once.

This has the upside that if he DOES pass his save, his attention will be on the crazy lizardman chanting and trying to inflict a spell on him, and in that situation you likely WOULD have the flat-footed advantage to work with. Correct me if I'm missing anything, but I think that works best in all regards- if it succeeds then he's dead with no issues, if it fails then you're in a perfect setup to catch him off guard since someone else has his attention.