This seems to be a silly idea.
Just because you have access to bitcoin through a mobile or portable device doesn't mean you are carrying cash.
You also have access to all of your fiat currency through online banking when you cross the border. The government has never thought of that as declarable cash.
Furthermore, if we keep our passwords in our heads. We can cross over the border without any electronic devices. We can then stop by any library and access all of our fiat "cash" by logging onto a computer. Does that mean we effectively have cash virtually stored inside out heads that has to be declared?
The whole idea shows how little cryptocurrency is understood here in the USA.
It is unfortunate that this law appears to be a fear reaction to what we don't understand. An attempt to control instead of trying to learn. This attitude is what leads to wars.
An old way that will not serve us well as we move towards more global communication, not less. Better get with the times and be on top of the learning curve or we will be left behind and lose our place.