As far as I know, Blizzard ignores non-standard tags that don't start with "X-". On whether or not tags are case-sensitive, I don't know and it would be something to test. If it is case-sensitive, you'd have to check each variant as there is no way to pass a pattern through the API to fetch the tag.
Is this for your LibAboutPanel rewrite? If an author wants to use LibAboutPanel, and they want their email to display on the panel in-game, then they should format/spell it the way you tell them to.
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh