It really depends on the Canton. Bern is very tough and insists on an MFK very soon, maybe Basel is easier.
There are heavy fines for driving without a current Ab-Gas test certificate.
If the Canton refuses to register the van without an MFK, you will have to rent the number plates from the Canton, on a daily basis, which includes tax and 3rd party basic insurance. http://www.baselland.ch/tagesausw-htm.288230.0.html