Dunno if this works with all/more browsers, but it does seem to work with Firebird (and I hope this is what you're looking for):

<a href="mailto:foo@bar.invalid?subject=Testing&body=Oh wow, body text
+">foo@bar.invalid</a>
[download]

This works for me, using Firebird and MozEX to launch Pine for me.

Test it here :) foo@bar.invalid