Paradox

Why not run the rifle down to the proof house in Liege and ask the guy who runs it what he thinks about your rifle and the pressure issue. He may tell you that you can increase the pressure enough to meet the minimum requirements and he could also re-regulate it for you to a higher velocity.

I don't know enough about smokeless substitutions to advise on that part.

As Mac says be very careful.

Sometimes it is better to not try to make a silk purse out of a sow's ear.