I have some of this functionality in a new custom block I wrote for piCal.

I copied the get_blockarray_coming_event function in piCal_xoops.php and created a new function get_blockarray_coming_event2. I added to this a new parameter "cat_filter". If you pass it a category id it will only return events with that category. The only other change to the function is to change the SQL query to add another WHERE clause.

I probably didn't need to copy the function but could have extended it in place by just adding a parameter, but I wanted to be safe.