OK, I found a solution that I think is going to work really well: The small
Blade Tech Molle Lok Attachment System. This system is designed to allow Blade Tech holsters to attach to a MOLLE rig, but it is perfect for attaching Safariland ELS receiver plates also.
Each kit comprises a pair of hinged, rigid plastic clips that slide through the MOLLE straps, and which have countersunk holes for mounting screws. A single clip can be used to secure an ELS receiver plate, so each kit let me mount two ELS accessories. The holes in the clip align perfectly with the holes in the ELS receiver plate, and the hardware provided in the kit is the perfect size to join the clip to the receiver plate without interfering with the installation/removal of an ELS fork/accessory. The photos below show how everything goes together.
What is great about this approach is that the clip secures through only a single set of MOLLE loops, making it very space-efficient. The clips are made from rigid plastic, and have a quick release closure at the bottom, so the whole contraption is quick and easy to install and remove from the MOLLE system at will. I plan to use these for a 3-Gun competition where "tactical" equipment is required; this will allow me to add and remove shotshell caddies as needed without otherwise disrupting my MOLLE rig.
I will post back here after the match, to report how secure everything proved to be under match conditions.