[SCRIPT3R]

to an extent, it is "stock" via queries. AdminCP >> Maintenance >> Execute SQL Query - for example,

TRUNCATE pm
TRUNCATE pmreceipt
TRUNCATE pmtext

will delete all private messages. more can be done via phpMyAdmin like...

EMPTY `PREFIX_pm`;
EMPTY `PREFIX_pmreceipt`;
EMPTY `PREFIX_pmtext`;

UPDATE `PREFIX_user` SET `pmtotal`='0', `pmunread`='0' WHERE 1;

but i see where you're going where you want to set conditionals. THAT would be an awesome hack.

P.S. here's my suggestion thread over at vB.com a while back. feel free to bump and let vB know that this is actually a valid request.