I'm a bit late, but...
One of my previous attempts was with been with matt medium and it didn't work much better than anything else so far.
I tried it with a Liquitex one available here. I thinned it with water. All my models are gloss varnished first anyway.
...ooh, yeah. I tried liquitex matt medium meself, for washes. It wasn't great. It stayed sticky, for one thing.
Alright for 2D painting, though.