Cummins Drift Truck a la Dum-Dum

[Poodle]

Can't you just give them a tap with a chisel?

They are so domed I think I'd struggle to get enough purchase and with the fact I've had to use my big breaker bar on the ones that actually have heads says I'm not gonna get enough force with the hammer and chisel.

That's kind of the opposite train of logic to how a hammer and chisel actually work, immense force is applied at the tip of the chisel. way more than your breaker bar, maybe even thousand of times more force. I suggest you try it. 
But sounds like you might cause damage elsewhere to the engine if you slip, so yeah, understand why you are being careful. 
Photo?

This, i've got all sorts of very tight and broken stuff out with a hammer and chisel before when all else has failed. There is a knack to it, maybe have a look on youtube if it isn't working for you.