Are any recreational sports mentioned in any fluff?

Scrumball, I think, in the Ciaphas Cain-books. It has really weird rules that to an outstander seem completely arcane, matches can go on for days, and it is only played in the Brittanica Cluster, iirc.

Gee, I wonder what real-life sport they were spoofing?

The Ciaphas Cain books make mention of scrumball (from the name, and context, probably akin to, or descended from, rugby). The Gaunt's Ghosts books have an unnamed impromptu kick-around*, which might be of scrumball, football (association, australian or american), or some other sport, as well as making mention of stadium sports results. There's what may be a similar kick-around in Execution Hour among the crew of a recently mutinied Imperial Navy vessel (on the other hand, it might just be an excuse to kick the severed head of a hated Commissar until it bursts).

The Ciaphas Cain books also mention Grasshopper, which is the one you were describing, SirFrog.

Multiple sources make mention of blood sports of one form or another (man vs man, man vs beast, beast vs beast, team fights, and pretty much any other permutation you can think of).

The Dark Heresy novels note that hang-gliding is occasionally done recreationally.

There was, at one point at least, a concept game being developed called "Eldavar", which was played by teams of eldar. Best described as zero-gravity netball, with a grenade on a variable timer as the ball. I rather enjoyed it when I played a demo game of it at Black Library Day 2001.

Add to that various races (foot, mounted and vehicle), with ork "Speed Freekz" being particularly fond of impromptu warbike/trukk/fighta-bomma races (with fighting between the crew/passengers of each. They are orks, after all).

*Offhand, I think it's in either Straight Silver or Guns of Tanith.

Blood Bowl, obviously

Thanks for all this.

I think I'll go for a type of medieval football. A large heavy ball is dropped into the middle of a cargo bay. The game runs for two hours.

Rules:

No killing

Ball cannot leave the bay

After 90 minutes the ball may be deflated and hid

The player(s) holding the ball when the whistle blows is the winner(s)