This topic really interests me for a couple of reasons. One, because I had Corona who is under 28" and also because I like, and breed for, the tiny ones and I often think of this issue with the colts I geld - which is basically most of the boys.
With Corona, due to his calm temperment and his winning show record in halter and also my opinion that he really has the apptitude to learn obstacle, I felt he'd make an excellent youth gelding so that is what I marketed him as. And I had lots of interest. I eventually decided on a 6 year old and from all the pics I've recieved I'd say it was an awesome match!
There are the 30" and under halter classes - I do not believe there are 28" and under classes for geldings in either registry but I may be wrong. But Corona (and Raven as another example) both were not only competitive but WON in those 30" and under amateur classes so why not Youth? And personally, I prefer a gelding for a youth (talking about a very young child, not necessarily a teenager) because I find them to be more even tempered and just easier, in general, to handle.
As for jumping, Brenda Sellwood has an under gelding (stallion until recently) that is an awesome hunter horse and has a winning halter AND hunter record at the world level, so I'd say why not?! Go for it!!
As a side note, I really had a specific type of owner in mind for Corona and I stuck with that, but I even had international buyers show serious interest. I think there is a market for quality under 30" geldings.
Good luck and let us know what you decide!