splitduty,
I worked in a twr from 2000 to 2008. Back then we had an internal document from our regional flow office (which management posted all around the twr) stating two things: The first one, that if there is no departure sequence all acft must depart at the exact CTOT time (can't make use of the -5/+10). The second one, that no acft can depart before the EOBT time, the explanation given for that was that the taxi time couldn't have a negative value. I don't remember if there was anything about it in the spanish AIP.
But I still haven't answered your question, because as I said before I'm not updated, I'm just telling how things were before but changes have been going on.