So, as printed, neither version of the Trick Arrow includes either the "Equip (Friendly)" or "Equip (Any)" keyphrase, and the errata for the Trick Arrow doesn't change this. Therefore, if the Trick Arrow is dropped during the game, it can't be reequipped (unless some character's effect allows them to do so).

I gather that there are a few other recent equipment (Broadsword Construct, Peacemaker's Helmet) that are similarly lacking the keyphrase.

Am I understanding this correctly? That is, I haven't missed any obscure rulebook update that gives all equipment "Equip (Any)" if they don't have one printed?

Based on the rules forum answers, it doesn't matter? They answer as if anyone can equip them if they drop.

This was noticed by ROC and they issued temporary errata for their events a bit ago:

Thanks. I went ahead and submitted it as a question to the WizKids Rules Forum as a "reminder."