i believe it is possible to use an #Include statement in the description.ext and point to an addon in a subfolder to use its internal scripting, if that helps
(Might be wrong here)
best way to make sure they have the addons is attach a readme with the pbo, and add the info in the briefing, then if at least one player has the addons, he can tell all the others what is needed
there was also a move in the past to incluce the "@" character in the mission filename as an indicator that non BIS addons were used
Some servers use it, some dont
eg
ctf@ 20 hold the hill v1.noe.pbo
The best way is to have good server management, an easy to find download section for that servers "Addon Pack" and/ or the use of OFP Watch as an auto addon update system for that server
Firrst line of the server's MOTD needs to be the addon download link, as this is always seen by somebody before they get disconnected, the further down the server .cfg file this line is the less likely they are to see it
Zeus uses it and it works great