Also, I haven't compared eurorailways fares to the Thalys site fares, but I wonder if they're simply building the ABS supplement into the fares they quote.

Just going back to my old post to answer my question. Found the answer on eurostar.com:

Eurostar tickets booked to and from Brussels Midi on the Eurostar are valid for travel to/from any Belgian station using the classic train service (not valid on Thalys). You do not need another ticket as your Eurostar ticket will cover the onward journey. You will need to take a connecting train from Brussels. There is no need to change stations as your onward train departs and arrives from the same station used by Eurostar

The onward journey is an open ticket and can be used on any train as long as the journey is made within 24 hours of arrival into Brussels.

The return journey must be made no more than 24 hours before your Eurostar journey.