To access C code from an interrupt context, all you'd have to do is:
.ISR: ; Interrupt service routine
movem.l d0/d1/a0/a1,-(sp) ; Save register not preserved across calls
jsr .C_ISR ; call the C-based ISR
movem.l (sp)+,d0/d1/a0/a1 ; Restore registers
rte ; return from the ISR