Intrigued by Eldargal's idea of linking the bastion, I blu-tacked some pieces together to investigate.
As you can see in the first picture, the walls of the defense line fit along the wall of the bastion just fine. The bastion door, however, is too wide and also flush with the ground to meet up against the defense line without modification.
[url=http://www.flickr.com/photos/47042253@N08/8200897179/][/url]
[url=http://www.flickr.com/photos/47042253@N08/8200897179/]Wall of Martyrs Bastion 1[/url] by [url=http://www.flickr.com/people/47042253@N08/]magickbk[/url], on Flickr
Exhibit B: The Door:
[url=http://www.flickr.com/photos/47042253@N08/8200897357/][/url]
[url=http://www.flickr.com/photos/47042253@N08/8200897357/]Wall of Martyrs Bastion 2[/url] by [url=http://www.flickr.com/people/47042253@N08/]magickbk[/url], on Flickr
In all, it wouldn't really require a ton of modification to make this work. One could just hack either the bastion door to fit the space on the Defense Line, or hack the end of a defense line to fit the bastion door. Each bastion kit only comes with a single door, though. One could simply make new doors for the bastion that would fit into the Defense Line end, which would solve the lack of doors and fitting problems. Making a little adapter out of green stuff/milliput and plasticard would also work, but still require some doors.
I think I would choose to make new doors, and add magnets to be able to swap between regular bastion doors, the scratch-built defense line doors, and armor plates or some sort of aquila to hide the magnet when using nothing.
Final verdict: Good idea, someone with some time get on this!