I think, just as unicycling companies have done with the ISIS hubs, they should do the same thing with seatpost diameter! I’m looking at frames to replace my lx and there are just tons of diameters. I’m setting the standard at 25.4 mm seeing as on udc, that’s what most of the seatpost diameters are.
their are lots of people who ride with different size seat posts and and they either don’t care what size or they ride with a given size for a reason. it would be immpossible to get everyone to ride with a given size because I don’t think Kris Holm would start making 25.4 seat posts/frames just to set a standard.
with the hubs, they are producing a new model each year, so they are constantly changing so it is easy to get all the brands to a certaint style.
but with the seatposts they are mainly only reinforcing them, not rebuilding them
You’re missing the point slightly, crank standards are necessary because to define a crank interface you need a lot of factors i.e. axle diameter, number of splines, spline profile, taper etc. Seatposts are defined by a single factor i.e. diametre so there’s no need for a complex set of definitions. In the same way that not every crank is ISIS, not every post is the same diametre, and a good thing too, different posts do different jobs, why on earth would you want to constrain a freestylist and a triasl rider to have the same size post, when their needs are clearly so different?
International seatpost design? 
I usually see 25.4 and 27.2mm seat posts. Those are two most common sizes I have noticed.
The reason there are so many seatpost diameters is that not all unicycles (or bicycles) use the same frame tubing and frame material. Some tubing has thicker walls, some is double butted, some is a stock size while some is custom. Different materials (different alloys of aluminum, different steels) need different wall thickness and diameters for the needed strength.
There is much in terms of engineering design and available tubing sizes that influences the choice of a seatpost size. Specifying a specific size to be the standard will limit the design choices available to frame designers trying to optimize their frame design for weight, strength, and other features. It will limit the material choices. It will make some unicycles more expensive because they’d need to use custom tubing just to get the standard ID for the standard seatpost size. Standardizing on a single seatpost diameter is not a good idea, especially for higher end unicycles.
Standardizing on a common size for the low end unicycles would be possible and we’ve already got that for the most part. Most low end unicycles use 7/8" (22.2mm) seatposts. 1" (25.4mm) is more recent and getting more popular. So there are two common sizes available that covers pretty much all low end (and many high end) unicycles. That’s reasonable.
27.2mm is a common size for aluminum frames. Aluminum frames need larger diameter tubing due to the nature of aluminum (strength, etc.). So you end up needing a larger diameter seatpost with aluminum frames. Most aluminum unicycle frames use 27.2mm seatposts.
So in the unicycle world the most common sizes are 7/8" (22.2mm), 1" (25.4mm), and 27.2mm and that covers the majority of frames. Some others (like Hunter) use different sizes. That’s more sane that the bicycle world where there are lots of other sizes in common use.