Could you add a broadcast alert (customizable) when the bosses spawn. This would alert the whole server they have spawned and where. For example this how I manually do it on my server:

cheat setplayerpos -155000 91000 11700
admincheat broadcast <RichColor Color="1, 1, 0, 1">Heroes of Annwn!</>\n<RichColor Color="1, 0, 0, 1">A force has entered the world on one of the islands near the green Obelisk!!!!</>\n<RichColor Color="1, 1, 0, 1">Dispatch this beast in your gods name to bring blessing upon your tribe!!</>
admincheat summon spiderl_character_bp_easy_c