Interesting. I just got my Gunbird 2 PCB last night, so I'm still learning the basics, but I'll have to try some of these techniques. About all I've done so far is repeat the first couple levels until I can find the gemheads pretty reliably. Honestly though, I think I need to practice coin chaining before getting into any more advanced tricks like milking
It's worth noting that the hidden character Aine has the same sort of bomb as Marion, it turns enemy bullets into 200pt awards (roses in this case). To select him in the PCB version, put the box on the random character selection and press the joystick down. I have no idea if he's in the console versions or not.