Polo Rules

Can I mallet fuck stationary players, as long as It's not a slash?

As John said, you can hook mallet-to-mallet but not slash. Hooking in the rules refers to mallet on body/bike.

§9.4 – Hooking
§9.4.1 – A hooking penalty will be assessed when a player hooks their mallet on an opposing
player’s bike or body.

Depending on what you mean by 'mallet fuck' it's worth noting that wheel on mallet intercourse isn't permitted:

§8.6 – Sweeping
§8.6.1 – A Sweeping penalty will be assessed when a player uses their front or rear wheel to make
contact with a stationary player’s mallet that is firmly established and planted on the ground.