With a burst of work, I finished the entire loft railing system.
The first step was to cut joinery on three 4x7 Ponderosa Pine posts.
The loft railing uses 2x8 rough cut, weathered, circular-sawn Douglas Fir barn wood boards salvaged from a 1905 grange building in nearby Palouse, Washington. These pass through mortices in the pine posts and are pegged.
The base of each post has an extension which will be bolted to the face of the Douglas Fir middle tie beam. I'll also attach the base of each post to the top surface of the tie beam.
I'll most likely oil the pine posts, which will be gorgeous.
I like the mixture of old vs. new wood, the rustic agriculture look, and the pegged, timberframe style of construction.
I debated whether or not to pre-assemble the railing system or assemble at the cabin. Either way, its a total pain. Pre-assembled, the railing is heavy and awkward to move (and lift up to the loft). However, it took quite a bit of effort to assemble the railing and fit the boards through the posts and get the whole thing square. Some of those joints fit very tightly. The idea of taking it apart only to reassemble it again up on the loft was not appealing.
I will probably just get a hand from a friend to lift it and bolt it to the cabin.