I solved my problem by using a string before the attribute. Instead of a "_" I now use "ops-".
For the debugs, I made my overlay very chatty because for my project I needed to show the overlay worked. And It was easier to have everything in the same debug file. Those debugs aren't essential, but they can be useful to see how everything works. I could simply add a parameter to the slapd.conf file to enable debug or not for my overlay. Would this solution be good for you?
Johan