A variable exists. It needs to be applied on the .radius class or the .rounded class. The border and the a needs to get the radius when the classes are applied.
$button-radius: $global-radius !default;
We added the correct CSS to make this work: https://github.com/zurb/foundation-emails/commit/b7c45e286265877e6c818075ab7d220c248e54f7

There is a question of how valuable this is in the framework though. You can't have a button with a border and a border-radius (the way the tables work). Given many email clients don't support border radius anyways, this may not be a valuable feature.
@tdhartwick @kball Thoughts? ^^
I just added border-radius on my project even though I know it's degrades on many mail clients. I don't think rounded edges are crucial if you want to keep the project smaller but I do think the option is handy and I imagine a decent amount of designers would appreciate the option since it seems to be a popular design choice Really Good Email - See Button Trends
I was just trying to figure out why this wasn't working. I guess my issue was that I have a button border _and_ wanted a radius on the button. I suppose I can skip the radius, but it would be nice to have if possible.
@EHackettTriCore 2.1 has this in - does it work the way you did it? We found having the border and radius together was not achievable but happy to see another way if you have it.
I guess we can close this now since it's in the codebase ;)
I've noticed that the radius class doesn't work when using a color class like alert on buttons.
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I just added border-radius on my project even though I know it's degrades on many mail clients. I don't think rounded edges are crucial if you want to keep the project smaller but I do think the option is handy and I imagine a decent amount of designers would appreciate the option since it seems to be a popular design choice Really Good Email - See Button Trends