It sounds like you already have a lot of options for clothes, but thought I'd put this out there. I did this in while in college for a totally different purpose, but it worked fine.

If you can sew... and need to wear a favorite pair of dress pants (or any other for that matter),

Carefully rip the lower inseam seam of the pants leg with a seam ripper.
Sew a strip of velcro along the seam allowances of both seam edges with a loose stitch - 6 stitches/inch or looser if you can.

This way you can get the pants on/off and then use the velcro closure to close up the pant leg. This assumes the cast is not so big that the pants leg can't be closed up and that you have 5/8" seam allowances. If the cast is bigger you could insert a gusset of some coordinating fabric.

This would be like having zippers at the ankles of your pants that you see on some winter sportswear.

Once you get the cast off, then you can just remove the velcro (remember that long stitch length you used?) and restitch the original seam. Now you slacks are back to normal. However if your seam allowances are bound together, or felled this probably won't work.