Yeah, I’ve seen that suggestion and tried it myself; the disadvantage is I don’t know that I can trust all browsers to render type in a consistent fashion. Relying on hardcoded line height values is less than optimal.
The only other option is to treat the button as a table cell and set it up with a div, formatting it for vertical center alignment. That also requires hard-set values, but for the height of the div itself (the dimensions need to match those of the button rect).
On the old Hype forums, this request first surfaced in 2011, and again in 2013. Both times it was said to have been wishlisted for future releases. I’m guessing it fell off the scopes.