Okay, I did a bit of testing without the CAN hitch whatsoever, found a few things.
1) Regardless of D0's pin setting, some CAN messages are getting through. They may be corrupt but I'm getting 3 recurring ids.
2) Carloop's standby mode is opposite to that described in the documentation of the module it seems to be based off of. LOW is "normal" and HIGH is "standby".
3) Even in standby mode, the channel operates at 500kbps and initiating it at 33kbps breaks it in mysterious ways (in my car, the system claims the traction control module is broken).
When I get home I'm going to pull out a multimeter and make sure that my messy soldering didn't accidentally connect pins 5 and 6. If it reads fine, then looks like what I'm going for can't be achieved. I'll also try it on a photon, though I’m not confident it will change anything.
Edit: Soldering’s fine.