> Will this work as a before insert trigger?

To emulate an "Insert or Update" as available in MySQL you certaily
can use a trigger that:

- Looks for an existing row
- Merges both rows (e.g. taking the empty fields from the existing row)
- deletes the existing row.

This might result in something similar to the "Insert or Update" but
for performance you should consider that it does duplicate the number
of DB-Changes.

It should be advisable to change the program's logic here to
explicitely check if an insert is required an run the insert or the
update respectively.

Elmar