Mark, the references I have found on MIDI seem to suggest meta event hex 59 for key signatures. It appears to have "sf" in a range of -7 to + 7 to indicate the no of sharps and flats and "mi" in range 0 to 1 to indicate whether a tune is minor or major. My version of Cakewalk appears to only consider major keys, at least for purposes of entry. The only one I looked at with that was the Blacksmith from our site where it is given as DDor - if I go to the Cakewalk thing to change key, the existing entry is C major.
I could be wrong but I would guess for example that DMix simply gets converted to G, etc. Again, I'm not sure but I suspect abc is pretty unusual in its recognition of the modes. Phil Taylor once sorted us out at folkinfo on getting all our abcs showing correctly using Barfly which is able to suggest the most likely mode for a tune but I'm not sure where we stand now - I know I for one am not capable of recognising most of them.