Are scripts allowed to put custom player info obejcts into players (with myinfo:InsertInside(player) and then read back with player:GetPlayerInfo(myinfo.name))? Or is the list of valid PlayerInfos jealously guarded by the server?

AIUI info objects are accessible.  The Guild System MT envisions will use info objects/objectlinks to file.

The loader looks for old B3 player_info and converts it to b4 objects like the quest objects.
The bank uses one named BANK_GENERAL
So as long as the object has a unique name it should work.

I know Cleo used it a year or so ago in one of his experiments and it didn't crash the server, still got the object in one of my local test characters.