quota_over_flag examples?

[E.B.]

I can't find any posts on this list for peoples using quota_over_flag

http://wiki2.dovecot.org/Quota/Configuration#Overquota-flag_.28v2.2.16.2B-.29

If my userdb is sql what would be best script to use in terms of performance?
(I mean if over-quota-flag triggers script every time it changes and the script
calls CLI mysql client isn't all this so expensive to spawn a new shell session
which spawns a mysql client?)

Anyone knows how to use this flag with postfix *making postfix send
special reject* "user over quota" note instead of plain SMTP reject??
Is an additional database lookup (restriction class?) unavoidable? :(

TIA1

PS Looks like it is tricky almost impossible to make postfix do rejects
based on this flag for aliases.  (Special query would be a little messy
for our schema but i dunno at what point postfix resolves aliases?)