There shouldn't be any. In the US you are technically a crew member on duty (meaning no booze etc.) when riding the
JS, even if it is offline at another carrier. The jumpseat is not a revenue seat, so no value = no tax. I know that the taxman will see the non-cash value (we are in Europe after all) and try to tax it. If the CASS EU legislation would contain some "crewmember/duty" element the taxes could be kept to a minimum.
All highly theoretical as first and foremost EU pilots need to want this bad enough!
CASS USA was implemented after 9/11...think about that!