How to use a custom function in deluge criteria in lookup field?

The examples provided only demonstrate comparing the lookup field values to a static value (as in "year == 2011"), or a zoho variable (zoho.adminuser...) .

How can I use a custom function or variable? I tried:

year == get_setting("current_season")

Where get_setting is a custom function I wrote, obviously.

This gets accepted by zoho as a valid criteria, but when its saved and I reopen the criteria, it's gone?!

Any help would be appreaciated.