I think a solution is to add using a hash of the number. A simple one is to convert to base-26 with A=0, Z=25 for which 1011 => BMX and 1001 => BMN so the callout would be the pressure value followed by the hash. The hash is calculated by the system on the ground that is supplying the reading. The nav system would get both values put in and check the hash for the value against the hash that is supplied. If the number and its hash don't match in the nav system then the entry can be rejected. Alternatively the nav system can generate and display the hash which is read back to be compared by the controller. The advantage of the hash readback is that if the controller misread the value in the first place a readback of the same misread value is avoided.
I know - FAA/CAA = a 10 year study and $20M in investigations, but computationally this is cheap and easily verified.
This sort of addition is what allows the internet and CD/DVDs to function.