What if you just install a hand-crafted DueSoon.ics file on your webDAV server? Here's a simple one (change the DESCRIPTION and UID fields to match a sacrificial event in your database):
Code:
BEGIN:VCALENDAR
VERSION:2.0
X-WR-CALNAME:OmniFocus Reminders
PRODID:-//The Omni Group//OmniFocus 1.5.2//EN
METHOD:PUBLISH
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTART:20090917T000000Z
DTEND:20090917T000000Z
DTSTAMP:20090903T201829Z
SUMMARY:Due: Test Event
DESCRIPTION:omnifocus:///task/g5c41etmHwt
UID:g5c41etmHwt
BEGIN:VALARM
ACTION:DISPLAY
ACTION:OmniFocus Reminder
TRIGGER:-PT5M
END:VALARM
END:VEVENT
END:VCALENDAR