Wait...wouldn't one way around this be to enable keyword filtering (a nice GUI for procmail through the Panel) and send all mail with DHSPAM in the subject line (which is what DH's SpamAssassin does, apparently) to a "spam" folder? Doing so would require the following steps:
1) Login to webmail.
2) Click on Options.
3) Click on SpamAssassin configuration.
4) Go down to Message Scoring Policy and Define Custom Policy.
5) Enter a sensible number (for example, 3 or 4) to be used as your filtering threshold in the "tag level" box, and enter 999 in the quarantine level (which should prevent your mail from being diverted to the DB).
6) Click Set Custom Policy.
7) Logout of webmail.
8) Login to the Panel.
9) Go to Mail > Keyword Filtering.
10) Heed the warning about a procmailrc (if you've already set that up, then you probably know how to edit it as I am about to describe), and click "Add [or Edit if you already have filters] Filters" for the appropriate mailbox.
11) Click "Add Filter for [yourmailboxnamehere]".
12) Change the filtering criteria to "When email subject contains DHSPAM".
13) Select "Move it to folder" and name the folder "junk", "spam", or something like that.
14) Leave "Execute this filter" alone.
15) Press "Add Filter".
16) Poof! That should give you a new Mailbox in Maildir/.[junk/spam/whatever] that you can monitor and read from any standard email client. I've never used Roundcube, so I don't know where to go from there.
Those steps should work...the only caveats are that anybody who sends you an email with DHSPAM in the subject will have their email sent to your junk filter (then again, how likely is that?), and that this will be a normal mailbox: mail won't be automatically removed (as it can be from the quarantine) after a certain period of time.