Offcourse you can... Otherwise they have to forbid you also to run other mods next to
PR:ARMA mod. Even that it doesn't physicly change the
PR:ARMA mod itself but changes gameplay mechanics. And i even heard some people say they allready played ARMA:
PR Beta and ACE Mod together. That than would probably also be a violation if you ran a dedicated server like that unpassworded. Only if a official
PR dev or other importend person tell's us that in these offical forums i willl believe it.
It's like Zimmer stated... If you want to play
PR without any mission changes (not map/island changes) (map and island are the same) you play on the
PR official servers. If you want to make your own host with cople mods loaded inlcuded
PR you will be able to join them to. Passworded or not.
In ARMA 2 alot of servers are allready running edited missions from other people. And they all have the desency to not provide them on ARMAHOLIC or wichever website. They do that because sometimes they want to add a extra ammo crate or add NV Googles in a mission they where not in. Or even fix a simple bug or typo in the mission they find anoying. And most of the time they even tell the mission maker so he can make a official fix.
They only thing you need to do to create a custom map in ARMA2 is editing a line in the init field of a unit in the editor, that looks like this: this addWeapon "M16". Thats how simple it is to edit a mission in ARMA2. Every 12 year old kid can do it.
I msyelf never ever seen a mission maker join a server and tell those people to pasword their servers because they are playing a edited domination or AAS. I think if a EULA like that would be viable there would be hundreds of missions out there with such a EULA.
P.S. They can do it like the berzerk guy. He scripted his missions so complicated nobody understanded how he did it

It was so diffecult to add a extra mod weapons in it (so you could choose it as the original mission from the berzerk menu) because than you had to change 30 other things, if i can remember correctly.