I have a VBA function defined in a module of one workbook. I'd like it to be
accessible to other workbooks open at the same time. Ideally, I'd like to do
this in such a manner that the other workbooks - whether opened before or
after the one containing the function - don't require any user actions in
order to access the function.

Is this possible? If so, how? (For reasons not worth going into, I'd like to
avoid taking the "add-in" route.)