I asked that very question today. Looking forward to replies.

Try using crushed walnut shells (the sort used for media blasting) sprinkled on to wet varnish, allow to dry and brush off any excess. Then apply a couple of coats of thinned varnish to fully encapsulate and secure the walnut shell grains.

Imaginative use of masking tape will produce stripes or any other shape.

The crushed walnut grains are available in different grain sizes, are inexpensive and readily available on eBay here in the UK. I'm experimenting with 0.5mm and 1mm grain sizes on my NE Dory and will report back when complete.

If you go into a marine store, into the paint aisle, there's a small can of non-skid additive (sand) that you can add to paint.  Not sure that varnish has enough shear strength to hold up to the scrubbing forces.  You can definitely add this to the last epoxy coat though before varnishing and it should still work.  Hope that helps.

   It may be heresy to some here, but I used Sikkens Cetol finish on the teak grates in the cockpit of my big boat for years and it never was particularly slick.  Now, I didn't use the Cetol Gloss topcoat, just the Cetol Light.  It does color the wood so if you haven't used it, you may get the smallest can and try on some scrap to see if you'll like it.  It's not magic, and some may not like the look, but done right it might be the ticket for you.  That said, I eventually removed it from all my handrails and went to twice yearly scrubbing and treating w/ teak oil, because the shape of the handrails is a PITA for finishing w/ Cetol or varnish. So much masking to do & cleanup.