So the thought process is to take your dirty oil mix it with our clean oil and ship it south at which point we will export it on your behalf.
Close. PADD 3 refiners (Gulf) crave heavy oil to process. With various by products of interest produced. Little to no dilution with US light oil required. That game is played with Venezuela oil which is pure goop. Ship US light oil to Venezuela, where they mix it with their goop, and ship it back to the US refineries. Of course Drumpf at last report had cut off US light oil exports to Venezuela.
What the Canadian oil producers also want is for their oil that is exported as is from US Gulf to end up through bureaucracy in a "Free Trade Zone". Exported from the US tariff free. Or as you suggest a pass through mechanism.
Someday the Chinese will pay to build, extend Canadian West Coast port infrastructure. Including the pipelines leading from the oil sand fields. Then we'll get to see if there's any concern in Canada about their aquifers.