As suggested in https://www.virtualmin.com/node/29763, I set Webmin's Users and Groups module setting "External password-checking program" to (tail -1 | /usr/sbin/cracklib-check | perl -ne 'print $_; exit(!/OK/)') and it works. If cracklib-check doesn't like the new password, it is rejected.
Unfortunately, it doesn't work in Usermin's Change Password module. Usermin accepts any old password, no matter how weak.
Also, the "Pass username and password to program (as input, as parameters)" setting is completely missing in Usermin.