I set up Dovecot Deliver and yes it works. I opened up port 2000 in my firewall, and yes it works from my local kmail client as well, but I'm probably going to disable it in kmail, and manage it completely from Roundcube, since Roundcube's interface is GUI driven, and kmail's interface is via programming; too easy to make mistakes, so I'll simply log into Roundcube to maintain the filters.
The only issue left for me is procedural: in the past I've had multiple email addresses and I've read them all using pop3, and further sorted using local filters. Now, to make it easier with sieve, I think I can use forwarders for everything except one main mailbox, then use forwarders for the other addresses. And use sieve to filter the emails directly into the folders in which I'd like to put them.
Thanks for a great addition to DirectAdmin!
Jeff