There are two problems here that share a single solution: (a) people without a valid disability using DAS to obtain an advantage over other guests (i.e. by abusing the system), (b) people with a valid disability using DAS to obtain an advantage over other guests (i.e. by using the system the way it’s currently designed).
In theory, DAS is not meant to give disabled guests an extra advantage, but only to remove the disadvantage caused by their disability.
I’m sure this would present some logistical challenges, but the best approach would be to have guests scan their MBs upon entry to the standby line, which would automatically trigger forfeiture of any current DAS LL reservations. This way, regardless of whether you qualify for DAS, you can only wait in one standby line (real or virtual) at a given time. Similarly, if you have G+, redeeming a G+ LL would result in forfeiture of any current DAS LL.
There would no longer be any incentive to abuse the system by feigning a disability, as it would provide no advantage. And those who can’t wait in physical lines would continue to receive the necessary accommodations.