Surely the same as now if you are IFR under CAS defined as an altitude, i.e. 'the QNH of an airfield under the CAS' or words to that effect.
And if you're flying in an area where the only controlled airspace above is the UIR above FL195?
If you're receiving a service from ATC in the vicinity of an airport, it's easy -- you use whatever QNH they give you. The tricky case is where you're talking to no one in the middle of nowhere.