This leaves tankers, which the Jones Act of 1920 requires to fly the US flag and employ American crews if they are ferrying products between US ports.
I would think this might be more feasible when the XL is up and running.
Why would oil companies import oil to Houston from a foreign port to then load on a US tanker to move it to another port?
They will simply continue to import the foreign oil to the different ports. I must be missing what point you are trying to make. What US port do you believe has a surplus of crude oil that could move it by US tankers to another US port?
I would think this might be more feasible when the XL is up and running.
The Keystone XL Gulf Coast Project would deliver an additional 700,000 BPD to the Gulf Coast. Later it will be upgraded to 830,000 BPD.
Gulf Coast Pipeline Project
http://www.transcanada.com/gulf-coast-pipeline-project.html
But the Gulf Coast already is importing 4.5~5 million bpd.
Gulf Coast (PADD 3) Imports by PADD of Processing of Crude Oil
http://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRIPP32&f=M
With this expansion they are still going to be importing ~4 million bpd. They don't have a surplus to ship crude to another US port.