Schmoe,

How much would that suck during a GPS approach to minimums far away from
the
crisis? Would WAAS stations be shutdown to?

One of the functions of WAAS (apart from accuracy enhancements) is integrity
monitoring. In the above mentioned case you would get a "integrity warning"
(in a predetermined time interval after the shutdown, IIRC 6 seconds) if the
satelites used for your navigation solution are affected, and should go
missed (or continue on an other nav-source, if operational and monitored).

Regards

Kai