I’m not sure to understand your proposal very well, but you should think about a possible protocol using new ucoin 0.2 transaction rules :
The mecanism with the XHX and Locktime parameters should be especially useful to build a DAB system.
The most difficult problem is the fact that the bill could be exchanged while being invalid. I guess, if it is simple enough to check the bill validity with a connection, people could trust the system if the company behind the bill is trustable ?