Great idea. I am in the area and I love going there.
Here are my thoughts (not in any order):
1) I think that with groups of 20 or more (and a minimum of 2 weeks advance notice) they will set up a tour for you (and it is free I believe). I have been on their general tours and have been impressed by the knowledge of the tour volunteers.
(By the way, does anybody work or volunteer there?

)
2) I think they post their upcoming exhibitions 3 months ahead of time, so we should check in February?
http://airandspace.si.edu/exhibitions/coming/3) I think that the date is about right. Weather can be good, the public and private schools are done with their spring breaks (museums get crowded during spring break), it is past tax day and I think a Saturday is a good choice.
4) The one issue that pisses me off at Udvar-Hazy is the parking fee scam. I wish that they would get rid of it and replace it with the regular admissions fee (which in DC is zero). If cost is an issue for anyone, they should plan on car pooling with someone to the museum (They do not allow walk-ins).
5) BTW a must download here:
http://airandspace.si.edu/visit/mall/floor-plans.cfm6) Did you know that there is a Hooters just down the street?

Just saying. or even better:
http://www.scout.me/breweries-and-brewpubs--near--chantilly-va