Interesting paper.I don't think we currently sell anything that will do that. Some of the old 7000 mainframes (ie 7834) allow you to output the A trigger signal, but not one minus the other.
Probably your cheapest option would be to get a 50 Ohm BNC relay from a third party.
Just FYI, our newest scopes have software to measure loop gain and phase automatically:
https://www.tek.com/en/video/measuring-bode_control-loop-response-of-a-power-supply