Hi Paul,
I have reviewed the image of your model. Your assumption is correct....for the fillet region you should be creating a WEDGE mesh.
The available selection types for creating mesh regions are datum point, vertex, edge and 3 sided surfaces (4 sided in case of BRICKS).
In the event that such references do not exist, you can always select surface/edge locations.
The mesh region points are connected along common edge.
Note: If edge does not exist, then common surface is tried and joined along it. If both of them do not exist, the points are joined via straight line.
Would you be able to share your model? I would like to give this a try.
Hope this information helps.
Regards,
Mark