It seems to me that the ultimate thrust of this proposal is to offer a
variant show-paren-mode.  This new variant would differ from the current
behavior of firing only when the cursor is located immediately before an
open-paren or immediately after a close-paren to a new behavior in which
all logical whitespace (including comments) on the current line is
ignored.  Such a conceptual model has a number of virtues:
- It relates the proposed behavior to today's behavior in a clear and
intelligible way.
- It points to a fully general model that includes how to behave when the
cursor is within whitespace inside a line.
- By accounting for _all_ whitespace on the line it finesses the need for
"periphery" or similar terminology.

/john