That’s going to be tricky in vanilla Tracery, to the point that I’d suggest stepping outside of Tracery. If you’re running Tracery via JavaScript, for example, I’d personally add a JavaScript function to reverse the words in the generated string (fairly easy with most string libraries) and then feed it back into Tracery.

The bigger principle here is that a single generator doesn’t need to do everything. Often it’s better to have a generator that is almost what you want and then add a post-processing step to clean it up. A recent example I saw was a archipelago generator that made really nice islands, but scattered too many small extra islands in deep water. But those tiny anomalies are easy to detect and filter out, so rather than fixing the generator I advised that the best way to handle it was to fix it in post.

If you want to do it in pure Tracery (e.g. for running on Cheap Bots Done Quick) I think you could do a complicated hack with the syntax for pushing values. Tracery has the concept of modifiers, so you could probably also implement a modifier that reverses a string or something like that.