If I'm only fighting the negative spring, I would think I could - certainly if 20psi in the positive chamber makes this difference, I would expect to be able to compensate for that 20psi in pulling it!
This is the bit that I find puzzling as all that you're trying to do is replicate the effect of setting a higher air pressure - if you bleed all the air out of the forks what happens? Presumably they'll be partially compressed by the negative spring, so will settle into their travel, but can they be fully extended by hand under these conditions?
I've looked at online service guides for these forks and I can't see anything to suggest that there is anything at work here except the balance between air pressure and negative coil spring.
I'd drop a line to firstname.lastname@example.org
and see if they can throw some light on the matter as I'm baffled, to be honest....
Incidentally, while mulling this over yesterday evening, I realised that I've never owned (or therefore worked on) any make of forks that doesn't begin with the letter "M".
So that's Marzocchi, Manitou, Magura and Maverick - strange......