You can create a library field named "Tooltip" and fill it in with an expression to display in the "Calculated data" portion of the field.
Then you have full control over the tooltip information that's displayed.
Wow! I've wondered about this for a while and had no idea [Tooltip] was available. Would you mind sharing the template that MC uses for the tooltip by default? By template, I mean that text that is displayed if [Tooltip] doesn't exist. I could try to work it out myself, but it seems have a number of conditionals in it. If it's not in expression language format and rather, in C++ logic, then I guess it's not worth the time to share it here.
My intention is to use the template as a place to start and then add, subtract, and rearrange to suit my needs.
Thanks,
Brian.