Just a thought

I can do it, not as fast as normal, but still pretty good. It's all about being able to recite blocks of the alphabet in your head (like, you know the song, usually you know blocks of it such as : 'a b c d e f g' ... 'h i j k' ... 'l m n o p'... 'q r s t u v w' ... 'x y z' and then you just revert those on the fly.

I can recite it backwards as well. Strangely, I had to learn how to do it as part of a Scientology course

I learned it backwards by breaking it up into
Zyx
Wvut
Srq
Ponmlk
Jih
Gfe
Dcba
Basically the same as lamortenmarche said but just shorter sections