Ray,
I suggest you talk to merchandisers up in oat country about getting Man/Sask/Alta oats hauled into Iowa, then they can be worried about getting the back-haul. Convertahaulers or tandem/tandem walking floors might be the answer for bulk down and cargo home.

I suggest this because over here in the Great Lakes area, there are lots of Canadian bulk haulers that run stateside, but very few US bulk haulers run in Canada. The US guys have lots of opportunity in 48 states, so when you start talking passports and customs/brokers/etc, their eyes glaze over and they lose interest real fast.
From the Canadian perspective, especially us here in SWOntario, if we want to do business with anybody more than 100 miles from home, in 3 directions we have a border to cross. We figure out what needs to be done, and get it done. The grain haulers that dont want to cross (dont want to pee in a cup, IFTA #'s, operating authorities, insurance) will have a hard time buying their lunch on the local rates. The trans-border guys generally do fairly well.

See about freight originating at the other end. I'm betting the 'list' is longer there.

Good Luck,
Ken