You can definitely do it.

I managed a 20" hardtail in my old mk2 any number of times, without removing any seats, and with a

(slimline) sub box behind the passenger seat.

It wasn't even that uncomfortable.

Both wheels off, seatpost out, frame in with the chainstays down in the passenger footwell and the bars/fork turned to whatever is most comfy, then fit the wheels around that.

Seatbelt it up to stop it shifting when you're driving.

You may want to put an old blanket down first, cos I was super annoyed when I ended up with dirty oil all over my seat bolster.
