I decided to clarify my question anyway
I make mission. In the description.ext I include weapons from bas_weap.pbo. However, no unit on the map is initialized using weapons from bas_weap.pbo.
If the player doesn't select a weapon during the briefing from bas_weap.pbo, the pbo is never loaded and never required for the mission.
If that mission is run on someone's computer that doesn't have bas_weap.pbo, a brief message will appear at runtime stating some weapon can't be found, and the mission will continue loading. All the weapons found in bas_weap.pbo will be removed from the briefing selection list.
No harm, no foul.
My concern is that someone will freak out about the message (no entry <xyzweapon> and state the mission is fouled up, when it isn't. I just want to give the player the widest choice of weapons possible.
All the addons *required* for the mission to play are included in the "addons" section of the .sqm file. Its just the "extra" addons that provide weapons for the player to choose from that I'm concerned about.