To be honest, this is the type of call that ought to be made by governments, not individually by private citizens/organizations. Either all conferences nationwide should be cancelled, or none, but that's not a decision any can make individually.
If this is a national emergency, let's take national responsibility and share the burden nationally. Have the government shut down conferences, if that's the right call, and use tax dollars to reimburse the conference organizers to a reasonable level, that they are also forced to use to reimburse attendees to a reasonable level.
Really, how is this any different from flooding or hurricane damage that gets federal money to rebuild? A national emergency is a national emergency, period.