None of the custom easels I've downloaded seem to give enthusiasm for painting - the original in-game one does. Is it possible to re-add this behaviour to the custom easels? If so, then how?

They were probably cloned from an earlier expansion pack. The functions on the easel are private not semiglobal and contained individually in each object. You need to plug in at least these these from FreeTime:

Paint - Process - Core Loop (100C)
CT - Active Hobby Interaction (1063)

Or you can extract and copy over all BHAVs but need to change the group number on all of them, which can be tedious. It can probably be done in the extracted XML if you are handy with text files. Or you can clone the base Easel, setting the group number to FFFFFFFF and transfer all the BHAVs into the custom package. Of course have backups just in case.

Lord Darcy has lots of information about updating objects to FT, as well as AL.

Updating objects to FT

Thank you.