On the verge of giving up with Masdevallia and Pleurothallis

[Spiffy]

This might be helpful for you (scroll to the bottom). If you're willing to water a bit more frequently, and setup a humidity drip tray, then this is a good way to go.
Isle of Portland Orchids - Growing cool orchids from South and Central Americas

Personally I've had some success with masdevallias in clay pots and a mix of sphag and leca, as well as open bark mix in plastic pots. During summer I have them hanging outside under a tree, then autumn I bring them into an unheated utility room under a grow light.

Also they're very sensitive to hard water. I use rain water and not much fertiliser. Too many minerals can also make the leaves go black.

Hope that helps!