Yes, touching/resting non playing fingers on strings can help stabilise things. I used to plant my little finger on the sound board which helped a lot with certain arpeggios. Note also my technique here:
http://www.dystonia-bb.org/forums/mwd/posts/2330.html..which is a nice way of weaning yourself off supporting fingers.
The trouble is with these things is that, like the 'latex glove trick', the initial boost/improvement you get from the proprioceptive feedback is great and its easy to get carried away and just play, only to find that pretty soon the FD has 'adjusted' to take account of the new stimulus, the novelty wears off.
It's what you DO with the novelty that's the important thing. Any change /alteration or temporary improvement is an opportunity to work with and not to be blown.
With your M finger I would work on P-M alternations on a single string - get a metronome out and work out what tempo you CAN reliably return it every time by total relaxation and a light touch, not by 'forcing' or interfering with what it wants to do. Try it legato and sometimes staccato (by planting early), try different strings, loud and quiet. All variety helps.
Even if this means one pluck every two seconds, do it. I worked for months on stuff like this to build the foundations to move onto more complex things.
If the rest of your hand works reasonably well in combinations not involving M, then do those, you can always introduce M at a later stage. Grab a copy of something like Giuliani's 120 right hand studies, which is fantastic re-training material (I use the Berg edition which is excellent and improves on the original).
Substitute PIAC for PIMA fingering, for example, and away you go.
