It is possible to use an enlarging meter or densitometer with pyro negatives. However calibration is a major issue and has to be done individually. To complicate matters, the stain produced by pyro changes with film, developer and processing and each combination will require its own calibration.
Because the stain color changes with film and processing, and the response of MG paper changes with stain color, and the effect of the stain changes with the contrast filtration, it is impossible to make an enlarging meter that will work with pyro negatives in the same manner as it works with conventional negatives.
We have many customers who use the meter with pyro negatives but they each use it in a different way as an aid to determining exposure.
If you do want to meter with pyro the following points may be of some help:
You will need to make a 'pyro step tablet' as a calibration reference. It can be made by contacting a standard step tablet on the film you will be using and developing it in your usual manner in the pyro of your choice. You don't really care what the densities are, you just want a set of patches you can use as a reference - you will be correlating meter reading to print density, so the actual density of the pyro-tablet negative isn't of any great interest.
When printing you need to do all metering at a fixed magnification and aperture. This is the same magnification and aperture that you used when you calibrated the meter reading to your pyro step tablet.
After metering the negative you would then move the enlarger to your final printing height, make a reading on the base + fog at the reference condition and again at the final printing condition and apply the difference to the base exposure.