The only way around it right now would be to use MessageBoxes! Seems a bit silly to use them, but it could be temporary.
I have an idea though...
In your installers that it extracts, add...
WriteINIStr "$TEMP\mysql.tmp" " " " " " "
...to the .onInit functions.
..in the .onUserAbort and .onInstSuccess functions of your installers.
Then in this main installer, add this...
exec "d:\mysql\setup.exe" #Executes installer
IfFileExists "$TEMP\mysql.tmp" loop
When the installer is extracted and runs, then it will write a mysql.tmp file to the Windows Temp dir. After executing, it will check for the existance of the tmp file, and while it is still there, it will not go to the next secteion, but will keep looping until the tmp file is deleted. As soon as the extracted installer finishes, or is canceled, then the tmp file is deleted. Therefore the main installer will go onto the next section.
Hope that helps.