This sort of sneaking in is pretty pointless though. The only really good reason to go to one of these schools is that everyone knows how hard is is to get in. If you go to a different part of the school that isn't as hard to get into (this is what GS is at Columbia) then you throw away that advantage and you're left paying three times what you should be for an education that, as was alluded to elsewhere in the comme…
I would say that while I loved everything I did while I was in school, it was the outside the classroom opportunities (student government/organizations) which were generally restricted to just undergrads that really taught me the most useful skills and connected me to a much broader network of people that I've found very helpful in my professional life. While I wouldn't say that it's a bad idea to "sneak in" I don't think it's the same experience/value as the regular undergrad program.