AFAIK it is always the field POPULARIMETER or POPM - only the internal format varies.
E.g. for MM the real, unprocessed format is:
5 stars, writes no@email|255|0
4 stars, writes no@email|196|0
3 stars, writes no@email|128|0
2 stars, writes no@email|64|0
1 stars, writes no@email|1|0
0 stars, writes no@email|0|0
Whereas Rating WMP takes
Windows Media Player 9 Series|255
Windows Media Player 9 Series|196
Windows Media Player 9 Series|128
Windows Media Player 9 Series|64
Windows Media Player 9 Series|1
To see the actual raw contents of the field POPULARIMETER, see File>Options>Tags>Enhanced.
If you toggle that option and you have 2 columns, one for POPULARIMETER and one for e.g. Rating MM then switching off the raw values will display an empty column for POPULARIMETER and a filled one for Rating MM. Vice versa, you will see the raw value in POPULARIMETER and nothing in Rating MM.
So it should be easy to write a set of actions to replace a single digit with the further info for the respective player.