I would go right to the source and ask Bear. He has or had that split tail mold and of course he has the injectors.
I just went and checked some of Bob's molds that I have, against Bear's medium injector. The 3.5" frog's sprue is a little too small for the nozzle. The 4" frog's sprue is large enough but you don't get the snug fit as with the molds intended for it's use. I, myself wouldn't try it as it's an accident waiting to happen..........for sure! A little too much pressure and boom, it's coming back at you. I've been contemplating the same thing because I like the injection molds I have of Bear's. The thing to remember is, if you do inject Bob's or any other "hand pour" molds, you still have to go back and "top off". With the single cavity injection molds, you can do the laminates and they do not require any topping off. I do believe one or some of the guys modified the nozzle (I don't remember for which molds but there is thread on his site) to fit snug and inject more safely with the "hand pour molds".