Borrowing handicap permit is already illegal, and AFAIR carries pretty hefty fine. Of course, enforcing it is hard - would you demand a spot physical endurance exam from everybody parking in a handicapped place? How would you even know? I used the temporary handicapped placard once for a while, when I broke my leg. When recovering, on some days, I could walk for a bit. On others, I could only walk with crutches and for a short distance. But maybe I was just faking, who knows? And of course I didn't carry my medical records with me everywhere I went. So how would you know if I'm legitimately hurt or just borrowed the placard?

If you are reluctant to harass legitimately disabled people, some amount of abuse would slip in. I think unless the abuse is so rampant that legitimate users get denied access (e.g. so many fakers are parking in handicap spots that the real ones have no spot) ignoring most of it as the inevitable price of not making lives of legitimately affected people yet harder is the right way to go.