Are altimeters calibrated to ISA geopotential altitude or ISA geometric altitude?
The TSO for Air Data Computers is TSO-C106. It specifies for the minimum performance standard "SAE Aerospace Standard 8002, Air Data Computer - Minimum Performance Standard."  

From AS8002:


  3.6 Altitude Reference:
  
  Calibration of the static air sensing mechanism for pressure altitude
  outputs (including baro-corrected if supplied) shall be to the
  geopotential altitude tables of U.S. Standard Atmosphere, 1976.


While this is a US standard, the FAA asserts it is in compliance with ICAO Doc 7488.  

For RVSM, additional requirements are levied against the installation primarily focused on reduction of static source error (SSE). That guidance is available in AC 91-85A.  

It's important to remember that absolute accuracy at altitude is not as important as everyone adhering to the same reference.