I made some trousers years ago from the standard Osnaburg linen. It just isn't a great choice for outerwear. Hard to sew (especially in the heavier weights. the thinner stuff is actually nicer to work with), and just doesn't hold its shape well. Fine for a hunting shirt, but just not so great for breeches. At the time, though, it was ALL that was available. Reenactors are a very small market group, so nobody makes stuff for just "us". I have even considered in the past getting a natural colored or tan/beige cotton denim fabric and using it. I would not fault anyone for doing so.
If it can be afforded, I would really recommend a linen or hemp canvas or twill... something with some "body". It will be SO much easier and more pleasant to sew, and make a better garment in the end. :wink:
Heck, I need to get my own breeches made! I have not been in the best of health for a LONG time, and everything has been put back. So many projects on hold.