How about a non-nil and non-t value of show-trailing-whitespace?

[Marshall, Simon]

I was looking for some way to turn on show-trailing-whitespace only for
files that are checked out of VC.  (I don't want to see loads of
trailing whitespace in files I didn't write and aren't editing.)  So I
looked at adding a function to toggle a buffer-local
show-trailing-whitespace to find-files-hook, vc-checkin-hook and
vc-checkout-hook.  Only there is no vc-checkout-hook, so I thought I
should suggest to Andre to add one.

But then I thought---maybe a more general solution is for
show-trailing-whitespace to have some extra semantics.

So, I suggest that redisplay is changed so that if
show-trailing-whitespace is non-nil and non-t, Emacs should highlight
trailing whitespace only if buffer-read-only is t.

How does that sound?  Simon.