Here's one where the only command used is QUIT

ClassMethod Encode(m) As %String
{
 q $$b
b(c="") q $s(c?1a:$a(c)-1#2,1:c)_$s($i(i)'>$l(m):$$b($e(m,*-(i'=$l(m)*i))),1:"")
}

Hey Paul! I knew I should have tested it before posting!

I like your very nice FOR loop. It solves the problem I had trying to get under 72

 //f i=65:1:90,97:1:122{s m=$tr(m,$c(i),i#2)} q $re($e(m,0,*-1))_$e(m,*)

F i=64:1:89,96:1:121{s m=$tr(m,$c(i+1),i#2)} q $re($e(m,0,*-1))_$e(m,*) //corrected