well, the major 6 is in your explanation too. Its the only minor mode (except for the melodic minor scale {and its modes} and from this point on in the explanation being ignored) that has the maj 6. Aeolian and phrygian have the m6 and the phrygian goes a step further to add the b2/9...
The dorian is the closest to major therefore it sounds happier too, regardless of the minor 3rd. All three modes, aeolian, phrygian and dorian have the b7. Playing C against your Dm would indicate all three modes could be used. However playing B (the maj6) and C (the b7) would indicate dorian, keeping in mind you have the other notes, or a few of them tagging along.
So in your improv's over m7 chords (or most other minors) you could use the appropriate scale (meaning the one in the key) or you could opt for the dorian. Opting for the dorian would keep it minor but also instill a happier mood as such.
All this depends on your chord in the end. If your chord contains the b9... it
may have to use the phrygian or a diminished or half-whole or whatever you deem fit that suits the sound.
Guess I waffled a bit too much there. I hope it clarifies
****EDIT****
It could be argued that using a dorian scale over a vi chord would change the key a bit... but it could also merely be used as a colouring tool... and a very effective one at that.
I slapped together two tracks for anyone wanting to hear the difference... its kinda subtle and my playing is shitty lately, so please bear thru this minute of crud
aeolian ---
http://www.box.net/shared/komvlpnx2y
dorian ---
http://www.box.net/shared/h2pj6gisc3