Waxing Spar Urethane finished product
Is it ... practical ... to wax urethane? 

Well, it can be done.  You would be ill-advised to do so before the urethane fully cures.  My stuff wants 30 days, but yours is surely 1-part urethane which cures via reaction with air or moisture, so I'd give it a few months. 

My concern is, I don't see how much good it will do.  Obviously you're very attached to the idea of "waxing" things, and I gather that is wisdom handed down over the ages. That is wrong-headed because coating technology changes much faster than that; all folk wisdom is obsolete.  If you were using a traditional varnish, then you should use trad maintenance techniques (but you wouldn't use wax for that).   But you went for urethane because either you wanted less work or more durability. 

That said, I don't see any harm to it... though when it comes time to recoat with an actual varnish, you'll regret it.  Wax is one of the four horsemen of the apocalypse when you're trying to prepare a surface for recoating (the others being linseed oil, silicones, and latex paint). It will require tedious, painstaking, complete removal with harsh chemicals, or the next coat will fish-eye and you'll have a ton of work to do.  I shudder just thinking about it.  

As far as resisting UV damage, polyurethane is already pretty good at that.  

Since you went after poly you presumably want a higher performance coating.  If so, avoid spar varnish.  Spar varnish is for spars.  Spars bend a lot, and normal varnishes will crack and split, admitting water, trapping it and rotting the spar.  So spar varnish has a lot of additives to make it flexible at the expense of traits you want in a varnish.  Not least, on normal brightwork you want a very hard varnish that will resist scuffing, because scuffing reduces gloss and gloss is the varnish's best defense. 

They make even higher performance coatings, but you will sacrifice "easy application" or "classic appearance", depending on the product you use.