There is probably some better way to do it, but this works too:
alias reverse var %x = 1,%r | while ($mid($1,%x,1) != $null) { var %r = $+($v1,%r) | inc %x } | return %r
Use as: $reverse(Hello world!)
EDIT:
Even better and faster alternative:
alias reverse return $regsubex($1,/(.)/g,$mid(\A,-\n,1))