Can a jacket keep a lizard warm?  

The answer to your question is No. Now the explanation.. :o)
The reason that Coats work for Humans, or all mammals for that matter is because we maintain a constant temperature. So the Coat works as a insulator to keep our heat IN. Since reptiles keep no constant temperature the jacket of blanket would only hinder it's ability to get warm. I would imagine that after a reptile is warmed up that a jacket may extend the effects, but the animals temperature would have to be monitored to insure the jacket or blanket was removed so the animal could warm up again sufficiently. Are you thinking about at night? Or just for trips outside?
-Tony :o)

I have never seen any body covering a lizard with blanket to keep it warm, but people cover pets like dog, cat , here one can see the cow and buffolo keepres cover them with rags, gunny bags..
Thanks for sharing

Happy posting, cheers.
Namastey.
Kalyani

Oh yes a blanket can verywell, a jacket too, if one covers them properly, but i will never lend my jacket to keep a lizard warm lol.
Thanks for sharing
God bless you, have a nice day ahead.
Professor ‘@Bhuwan@’. .