Entheojen said:
I don't see where the problem would be. Coffee ground for espresso machines is usually very fine, similar to powdered MHRB.
Contrary to the above statement, there are problems. I have tried the simple macchinetta espresso machine and it clogs bad time with powdered mhrb. It clogs that bad that the whole powder becomes a tough as a stone from the degree of compression it undergoes.
Rue works better, but not amazingly at all. Whole rue runs fine but it literally extracts 0% of its potential yield. Ground rue extracts ~40% of its potential yield in the first run, not very sufficient at all.
Espresso machine extraction sounds really good in theory and I fiddled with it a lot at some point, but in practice it is a very inefficient method.