This website is from Connecticut, but it probably explains the same thing:

"A sample dilution rejection will be issued to motorists whose vehicle emissions cannot be properly tested because too much air is present within the exhaust sample.

The emissions testing equipment automatically determines if exhaust gases are being diluted before and during every test. It does this by verifying that enough combustion byproduct, carbon dioxide (CO2), is present in the exhaust gases. If there is not enough CO2 present, the computer will not allow the test to proceed and the vehicle will be rejected."

http://www.ctemissions.com/expect-dilution.html

What's the full exhaust setup like? What kind of header, what diameter pipes, etc.

Assuming you have a properly functioning cat and no exhaust leaks you should be able to pass no problem. Obviously a gutted cat won't work.