Hitting Metal Sonic with projectiles

I'm working on a projectile-focused character, and I want him to be able to hit Metal Sonic without needing to jump or dash into him. The problem is that Metal Sonic's behavior causes projectiles to pass right through him, I'm assuming due to the lack of MF_SHOOTABLE. I'd like to know if there's a clean way of getting projectiles to register without interfering with Metal Sonic's original behavior.

As far as I've seen on the wiki, Metal Sonic is really hardcoded into the game, its damn near impossible to recreate and modify Metal Sonic. By hardcoded, I mean there are NO actions that can be used to modify.

This is why Lua exists, you see.

Just to help out, Metal Sonic's main boss thinker can be entirely overridden or added to with a BossThinker hook, for starters. In the meantime I'll check out the code for it and see if I can give you anything more useful or not later.