First of all, I think they key distinction that you have made, is that you are "Teaching them to play the patterns" of modes, which I entirely agree with. Once you know all the patterns you know all the patterns for every mode.
Your thoughts about using the Dm in a predominant role in a progression for purposes of training the student, and looking at that in Dorian, is at the very least...intriguing to me. I cant say anything more about it, because Ive never heard what that sounds like when you teach it, it would be cool to be a fly on the wall when you do this.
I have used the Dm in a vamp, in the sense that Might do Dmin to Dmin 9 To Dm7 etc, but all witht he central tonality of D so that the resolution feels firmly placed on D.
I've also taken the D minor blues and added a B note, ala Carlos Santana, over the Dmin chord.
Ive used the same idea to demonstrate how quickly the natural inclination to jump to C is "Hijacked" and how the finality of the Dorian is "lost" in that instant. Your approach seems to say, push ahead, ignore that finality and focus on the chord tones of that Dm stubbornly...and that's what I find intriguing. I may try that, just to see if it sounds "together".
Whatever you do, that approach seems to work for you, and your students benefit, and thats the important part. I just wish I could hear what that music/improv sounds like, when you teach it
Instructor