I haven't tried injecting anything into a mouse, no. I have one of the die-hard mousers though, and I've basically just ended up feeding him as many mice as he'll take. And I get your frustration, because he has grown MUCH slower than my rat and ASF eating snakes.

My concern would be that if the "supplement" was very far off from what they're used to consuming, it may cause some digestion difficulties since the gut flora are adapted to mice. And I have no idea where to buy tubs of rodent fat.