And if you can get the thread waxed, simply pulling it with all your strength should give you a fairly reasonable idea of whether it will be suitable. Of course that depends on application and the number of strands in the waxed end. But properly waxed and of good fiber, you shouldn't be able to break it easily...and certainly not during inseaming.
I would say, that I seriously doubt anyone alive has had experience with true premium long stable linen or hemp yard. I have seen linen being spun some of the strands are up to three feet long. I have also bought pre-WWII linen yarn, imported post-WWII linen yarn from the great Irish linen mills as well as purchased new manufacture yarn from places such as the link you provide (linen and "genuine hemp"

Tight Stitches
DWFII--HCC Member
[center]Without "good" there is no "better," without "better," no "best."
And without the recognition that there is a hierarchy of excellence in all things, nothing rises above the level of mundane.[/center]