I don't know of any rules around propegating an endangered species. I would think any devoted arborist would say knock yourself out!
I also have a species of Madrone that is native to WA that I would love to play with. But I've been told the problem with madrone is that the leaves resist the process of reduction. Thereby making it an ill suited species for bonsai. You don't want to do all that work for a tree that will not go along with the program.
But given their stunning characteristics as full grown trees I can understand the urge. One of my prize trees in my yard is a large madrone that I have been protecting since it was a sapling. And if you'd seen all that changes over the last 13 years to my place you'd know that was quite an effort!
Regards,