Just to put a full-stop under this one, I was almost there with my original assumption that feeding a hicon source into the effect matte would give the right result.
Where I came unstuck was that the effect won’t work at all if you don’t have something – even if it’s blank – feeding the background of the merge node. That, and making sure that the Channel that it takes transparency from is the luma channel.
So if I try the following:

Then the matte doesn’t work.
If I do the same, attaching a blank text node to the background:

The matte now works, and creates transparency on my foreground.
Hope that helps someone who’s in a similar situation!
Thanks again to those who helped!
Chris