I've never fed any transgenic mice to my snakes. I believe that whether or not it is safe would depend on what transgenics were used.
My only experience with transgenic mice is what I have learned from various primary literature articles that I've read for biology classes. Some of the genes that are used would make basically no difference in feeder quality. Other genes cause over expression and build-up of certain hormones. There are probably numerous strains of transgenics that exhibit qualities that may not be desirable in a mouse you are going to feed to another animal.
The bottom line for me would be that I wouldn't feed any transgenic mouse unless I knew exactly what it was transgenic for and what that gene caused, and that there weren't any other major side-effects caused by the transfer of that gene. Some experimenters do fairly wide ranging tests of their transgenic strains in order to make sure that only the expected result is taking place. For instance, if only result "x" is expected by inserting a new gene, then they would do a battery of tests/observations to make sure that some other unforeseen phenomenon isn't occurring in addition to "x". This is usually done just as a part of making sure that any results obtained are as valid as possible and can't be refuted.
I don't know...those are my thoughts on it.
:shrugs: