After being sold a manual RGB scart switch with no RGB connections and looking at automatic scart switches which were expensive, bulky or unnecessarily complicated, I came up with this...
The primary input (eg. set top box, DVB-T/DVB-S tuner) is output by default, whether the box is connected to its power supply or not.
A secondary input (eg. DVD player), controls which RGB signal is output with its switching signal.
The circuit can be built into a modified non-switching 2-way scart splitter box as shown.