Looking at the API explorer on the demo site it appeared that it's possible to query custom data on activities using the API.
However it seems that while the UI helps you select custom fields and set the values you want to search for, and there's no error submitting that, it seems completely ignored in terms of the query run.
I added a custom field set to Meeting activities on the demo (4.6.8 at 21 Aug 2015) and added a meeting activity with
foo in a custom field, then queried this using the API explorer, but it seems to return all activities regardless of the custom_N query.
The developer documentation says
As with normal API parameters, you can set custom_22 = 'Fred' in a get operation to filter for records that have custom_22 ...
So is this a bug/unimplemented feature or am I doing something wrong? Is there a work around or do I go back to writing plain ol' SQL?