Before: my client's linen pants had holes and worn fabric in the seat.
After: I patched the seat using fabric I had "harvested" from elsewhere in the pants. Can you guess where?
Before: the pants had two patch-style pockets on the back. This is a picture of one of them, before I did anything.
After: this is one of those back pockets AFTER I had done the mending. Looks the same, right? Actually, not quite. I sewed the two back pockets shut at the top and cut away the pant fabric underneath. I then used that "harvested" fabric to patch the wear/holes on the seat. The result was faux, rather than functional, back pockets, but (as you'll see below) an unobtrusive mend.
Before: this is a "before" picture, but it looks pretty similar to the "after" picture, which is the point!
After: Once I had done the "sew pocket closed and harvest the underside fabric" trick, the pockets looked exactly the same as before, but became faux rather than functional. The harvested fabric became exactly matching patches on the crotch/seat. Fortunately, these pants also had side pockets, so my client was not left pocketless. And I did check with her first to make sure she was okay with sacrificing the back pocket functionality, which she was.