Agreed that ` is not best and should be changed.  An alternative should be in next release.  I looked back at the output in msg268406.

! is similar to | but shorter, and better for that.  It is also, generally, thicker, which is better. With the font used on this page in FireFox, at my usual size setting, |is 1.5 pixels, so it tinged red or green depending on whether the half pixel is to the right or left.  ! is 2 pixels and black, which is better.  Smaller font sizes could reverse the situation, but unlikely for me.

@ is email separator and twitter name prefix.  Firefox recognizes this and colors the first @ and all names blue.  I don't like this.  Not an issue in code editors, etc, but code and especially results, get displayed elsewhere, as here.  Aside from that, it is visually too heavy and I cannot avoid reading @ as 'at'.  Let's skip it.

I still like my previous 2nd choice, ^, but you apparently do not.  I omitted ? before, I think just by oversight.  Let's try both with ! also, isolated from other options.

>>> for c in "^!?":
	print(".%stoplevel.%sframe.%sbutton\n" % (c, c, c))

.^toplevel.^frame.^button

.!toplevel.!frame.!button

.?toplevel.?frame.?button

? strikes me as slightly too heavy, but worse is the semantic meaning of doubt, close to negation.

With the noise of other alternatives removed, and looking again several times, I like ! about as much as ^, both visually and semantically. Perhaps from knowing some Spanish, which uses inverted ! to begin sentences, I read ! as mild affirmation.  I would be equally happy with either.  If you prefer !, go with it.