There is no mechanical reason you couldn't fit a pressure gauge in the steam line, plenty of industrial applications have pressure indication at several points away from the boiler. But, they all have pressure indication at the boiler as well.
There is a BIG however though:
Putting the PG in line means you will not be seeing the true pressure inside the boiler, as is required for certification.
The stop valve on the boiler could induce a small pressure drop as the steam flow crosses it, and, depending on the length of the steam pipe, there is a possibility of a further small steam pressure drop created by resistance to flow.
Ian