Postfix uses no longer MySQL to ask for data, but flat files.
Is there a chance that ISPConfig produces those flat file on demand? I think it was like this back in IPSConfig 2...?
Youcan write a plugiun for that. But we dont plan to change the postfix setup in ipconfig 3. I really understand that you have a problem there with your installation but if there are thousands of installations which are much larger then your setup and none of these have this problem, then there is no reason to change the setup.
So instead of talking to change a etup which has prooven to work reliably an many systems, you shoul try to debug your mysql server by enabling mysql logging to see why postfix is not able to connect to mysql.