Try to add these two values in preference file of icewm.
CenterMaximizedWindows=1
CenterTransientsOnOwner=1
If things still don't work, the only way will be modify
the source code of icewm.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Logged In: YES
user_id=1110932
Originator: NO
Try to add these two values in preference file of icewm.
CenterMaximizedWindows=1
CenterTransientsOnOwner=1
If things still don't work, the only way will be modify
the source code of icewm.