Do flat flexible connectors (FFC) couplers exist? (i.e. I don't want to use a cable)
Perhaps you could modify your daughter boards to be made of flexible circuits (with possible solid areas if required) so that the daughter board thin edge is the actual flexible flat cable.  You could then position them at suitable orientation and proximity to the main board and have a measure of alignment and vibration tolerance in the placement.

You could get ideas from the following Google picture search.

EDIT:
This link has a picture that illustrates what I had in mind.