I would most probably do one of these things:

1) Drill/tap the muffler, insert pressure fitting, and run with muffler pressure.

2) Purchase an aftermarket muffler that already has a fitting, and run with muffler pressure.

3) Drill/tap the backplate, insert pressure fitting, and run with crankcase pressure.



That said, I've run with "no pressure" before. (An older K&B, oddly enough... with a Perry carb)
I ran the pressure(or in this case, "vent" ) line through the firewall as normal, then downward
until it was just about flush with the bottom of the fuse. I used a forward facing 45 degree angle cut
on the hose, to lessen the chance of having a vacuum pulled on the tank while flying, and to lessen
the chance of fuel getting sucked out while inverted.

Never had a moment's trouble out of that setup.
I believe the forward facing cut probably provided a little positive pressure from the airflow, and
combined with the suction from the carb, I doubt I ever lost a single drop. (If I did, I never knew it)
You can always try it that way, but I just feel better with a more positive source of tank pressure.