I've be requesting video aspect ratio fiddling for ages. Even if it can't read it from the files (which if WMP can't, it probably won't be able to), it should allow you to specify it like this:
Aspect Ratio -> Original aspect
Free resize
4:3
16:9
1.85:1
2.35:1
Custom by image aspect ratio ... -> ____ : ____
Custom by pixel aspect ratio ... -> ____
The "Custom by image" option should allow you to type in two numbers to be used as the image aspect ratio e.g. 16:9
The "Custom by pixel" option should allow you to type in a floating point number for the pixel aspect ratio.
These are usually:
1.0 Square pixel
1.06666 PAL DVD/DV
1.42222 PAL anamorphic DVD/DV
0.9 NTSC DVD/DV
1.12 NTSC anamorphic DVD/DV
Ideally, these settings should be remembered on a track-by-track basis.
(If you wanted to get really flash, you should allow the user to set the pixel ratio of their monitor. Then I could use my widescreen TV in Full mode and MJ would squash/stretch playback to fit. Then it really would be the ultimate media player.