Generally, dismounts are allowed in most everything except Track races. With exceptions. But you’re always allowed to dismount in XC and Downhill MUni. Uphill usually depends on course length. Longer or harder courses usually allow dismounts, but short ones may be more strict because they are otherwise easier. Our early examples of Uphill races were often 100m or less so no dismounts. But the Unicon XVI Uphill was several hundred meters up a ski slope at around 5000’ elevation, on grass, which would be quite a feat for most mortals to do in one shot. I actually made it up with only one UPD, but I made several stops to “re-oxygenate my brain”. 
This is the traditional way of doing those events; to make the groups more or less proportional to the actual attendees. In other words, the object is not to reduce your pool of competitors, but to challenge yourself with a group that is hopefully of similar physical possibility.
Blue’s quote brings up the question of how does a rider know which set of rules will be used for a given event? A look at the NAUCC web site does not seem to turn up any information about that, or a link to the USA Rulebook. Neither does the USA web site, when I looked just now. If it’s on there, it’s not in an obvious place. Oh, I see you provided a link below. It’s listed under Business (but not searchable under “Rulebook”). That’s dumb. I did not find it there on my own.
What’s needed is easy access to the USA Rulebook, for starters. Also if there is a new version nearing completion, that would be good to tell people as well. As we’re into Spring, it would be nice to offer a summary of what has changed, so riders can train accordingly.
The host should (I think its a requirement in the USA rules) make it clear what rules are being used for what events. In the past it was just the USA Rulebook, but now that the IUF rules are being referenced more, it gets a little blurry. You really need to list this out, as it will save you a lot of grief between now and July as people start asking a ridiculous amount of questions. Generally you want to be able to refer people to the Web for the answers to as much as possible.
Mostly because if you do the same thing with a ring, for example, it can end very badly. 
History: The IUF rules are derived from what the USA was using in 1984, when the first IUF convention was held. IUF used meters instead of yards, but much of the rules were identical. Things have diverged over the years, with innovation coming from both organizations, but in recent years mostly from the IUF. Today’s USA Rulebook should be a reference to the IUF rules, with unique sections in it only for the parts that are different. A little messier to read, but if packaged properly it could be very easy to handle.
While IUF conventions started out about the same size as USA ones, now they’ve grown a lot bigger, and a lot more international as well. Different requirements for different needs, as Goat indicated. But also, each country may have its own traditions, and want to do things their own way, or in traditional ways that pre-date the IUF. The IUF doesn’t want to force the world to all go by a single set of rules if it would mean a loss of innovation. We like new ideas.
Goat, what will also help you as the main organizer is to have people you can send registrants to if they have questions about events. The people in charge of those events are usually best-equipped to give accurate, binding answers. If you have them picked out, put them to work.
BTW, I can help, though I’m still trying to figure out my trip out there…
which IUF winks at;)
Sorry to disappoint, but I’m only referring to actual earrings in ears :o
well…if you have the ones as specified in the amendments, it’s okay if I am the only one in my group ;), I would have a chance to place, 
Just messin with you.
It’s not a fun surprise.