I'm sorry to open a "bug" report for a support issue.
I've been reading through the documentation, and I know that CAN FD support is present.
But I can't seem to find out of it's supported via serial over CAN (slcan)

I know this is an ASCII protocol for traditional CAN:
http://www.can232.com/docs/can232_v3.pdf

But I am having a hard time finding definitive information about this protocol and CAN FD.

Edit: I think I found the answer
https://github.com/hardbyte/python-can/blob/develop/can/interfaces/slcan.py
This code doesn't support CAN FD messages, so I guess it's not supported directly.