What's matter with you? these fine people gave you straight advice about what size tire you can put on a 13mm rim and confirmed that info through Sheldon Brown and you laughed it off? Sheldon Brown is not being conservative by any means, there is a limit as to how wide you can SAFELY go with out causing problems with the tire; and with your 13mm rim that would be 25 with a 28 being the very largest size-if that manufacture of the 28 is actually closer in size to a 25-no one manufacture uses the same size measurement criteria.

If you want to cyclecross your going to need wider rims such as a 15, 17 or 19mm rim if you want to put on about a 30 tire. Mavic makes cross rims and Open Pros are not those. An inexpensive Mavic is the Crossland. Check with your LBS since I don't cross and therefore not sure what rim manufactures there are that make those types of rims.

But hey, I don't care if your going to do your own thing, but then why ask for our help?