I was just coughing up something else and stumbled across this assembler, which was previously unknown to me. According to the documentation it also generates code for the MEGA65:
vasm is a portable and retargetable assembler to create linkable objects in various formats or absolute code. Multiple CPU-, syntax and output-modules can be selected.
13-May-2021: vasm 1.8k.
6502: Implemented a few 45GS02 instructions (Mega65).
Due to lack of time I do not get to test right now, but just wanted to mention it here.