Bit of an open question but....
Is it possible to design a form in qt designer that will be used as a form against a qgis layer but interacts with a different table in postgres/postgis as a lookup table.
For example, I have a postgis layer that i'm editing in qgis to capture the locations of shops. Some of the fields to capture are address, postcode and parish. Could I do a lookup to another postgres/postgis table to enable these fields to auto-populated my shops form field?