The real issue is that all paintball ammo is basically the same weight, so for them, velocities are gold. There's rarely mention of muzzle energy in paintball. So to them, 300 is 300. There's nothing else. It IS based on weight and muzzle energy, but without the variable of multiple weight ammo, then fps is all field owners and insurance companies have to worry about.
In airsoft, we have various types of BBs in different weights, so we speak of fps with .2g BBs, but that being based on muzzle energy and the weight of the BB. And we know that under 400 with a .2g projectile is safe.
I wouldn't normally recommend this at an airsoft field, but if we wanted to get by their restrictions, we could get by their FPS rules easily by just chronying our guns with 0.3g BBs. That would put most well under 300. We know they're safe and within regulation velocities for airsoft with 0.2. If all they care about is velocity and not muzzle energy (VERY wrong, but that's what they insist), then we can give them what they want without affecting our guns. We chrony with 0.3 and use whatever we normally use for the game.
Again, I wouldn't recommend people do this at a proper airsoft game to get around regulations, but it could work at this field and still be safe.