SNK rotary sticks won't work for that. MAME interprets the input like a joystick (U/D/L/R and intermediates) for Front Line. Through an interface, MAME interprets SNK LS-30s as one of two button presses (L or R, sort of).
If you really want to do this, check MAME and see if you can configure Front Line to accept 2 button presses for rotation like you can in Ikari. I think I've checked already, though, and you couldn't.