Is there any way to do this?

Great!

Just so you know, I edited my post above after you replied.

By the way will it be available for primary weapons? Because I feel it's a bit stupid for a Private to be allowed to carry a grenade launcher, but not a LAW.

Absolutely, the rare weapons in invasion mode will be mostly primary weapons.

I suppose I now have an idea how to make it all work, the simpler the way the better: invasion mode will have its own overlay.

This will allow invasion to have any amount of weapons without the normal mode having to carry baggage about weapons that are not usable in it, normal mode weapons can be defined with 0-rank limitation (except LAW and grenades with their capacity progression), invasion mode weapons can be made to extend normal mode weapons by overriding the rank limitations in them easily and define several completely new ones. This way invasion mode could have map specific weapon commonness factors, map specific enemy factions and so forth.

As most of this information lies in map_config.xml which is just a short set of definitions, it could be even served from the server to the clients at the map change or when connecting pretty easily, allowing the invasion mode server to control / randomize rare weapon appearance in the battles, enemies and so forth, anything that can be controlled using an overlay and the resources the clients already have.