You could include a small Python script block with each label field.
This script could:
The API is especially useful, if you want to handle a field differently when another field has a special value, when you want to apply some calculation with the value, or when you want to use some field formatting not supported by the base system.
Your script block is executed with 2 local variables accessible:
Simply overwrite val
Label scripts can change variables by calling:
if val==1: val=5