heads-up, this is not a complete answer; it might help you get to the solution though
Your linux environment is perfect of
Phil Harvey's ExifTool
The stand alone tool might have a way to be scripted to do this.
One dirty trick is to use timestamps and bracket bias data to collect images.
There is a Perl library too.
Also see webhdrtools which is based on ExifTool & Perl among other things.
While I have not checked if it handles your interest of isolating bracketed images,
if it does not, take a look at the exiftool output for bracketed and non-bracketed images you might find a simple method to isolate them.
Finally, when you get this done, do share your work-flow here.
This has been on my road-map for a while too