For users on HostMonster hosting, I can verify that MrM's 0.1 stable password changer does work with one minor adaptation after installation.
You'll need to have ssh turned on for your domain before you can run the commands given in the installation instructions.
Then in file passwd.inc which is created by the patcher in program/steps/settings/passwd.inc inside your roundcube directory
edit line 43 to read:
curl_setopt($ch, CURLOPT_URL, $domain.":2095/webmail/hostmonster/mail/dopasswdpop.html");
ie: change x to hostmonster