Hi,
you can still use the default quota to achieve this. you need to remove all the other quota entries in the /etc/quotas file except.
* tree@vol/volname/ size_gb - - - -
this means that all qtrees in this volume will have the same quota - i.e. the default quota above.
e.g you have qtree1, qtree2, qtree3 in vol1 and the only the following entry in your /etc/quotas file.
* tree@vol/vol1/ 10g - - - -
then qtree1,qtree2,qtree3 will all have 10g as quota.
Hope this can help.