Back-up (pressure) altimeters would have local sea level air pressure value sent to them automatically by data link (with a readout of this figure available for cross-checking against forecast values). In commercial aircraft a discrepancy between the GPS and pressure altimeter indications exceeding pre-determined limits would generate a crew alert. Development of this sort of kit is within the bounds of current technology.
The Cub pilot would be responsible for manually setting LSLAP on his or her old tech altimeter.