The treads of my ladder are unfinished teak. Only the rails are varnished. Bare teak is pretty non-skid on its own. Sober, I've never slipped.
1988 PSC 34
I refinished my 1961 companionway a few years ago. I think the tread design is brilliant.
The treads, mahogany, are routed a half inch deep with a 1/2" round bit, every inch and a half (or so).
Each slot is fitted with a 1/2" by 1" strip of teak(I had to replace the the outer worn teak strip on each tread - they were very easy - sand the ends to fit). The strips are tacked in with counter sunk brads(they can't move in their slot).
Here's an even easier step: After stripping the ancient varnish, I simply revarnished the whole thing.
When it was dry, you take a block with coarse sand paper, and clean the varnish off the top of the teak strips. Done.
If treads are removable, this would be an easy add on.
We've (a family of 4) had this boat for nearly two decades, never a slip on these treads.