I just bought a Richard Sachs UOS fork kit, with plug-in style drop outs.

Since the drop out inserts inside the fork leg, how is brazing material fed into the joint? At the gap between the drop out and fork leg?. Add a notch on the end of the fork leg? Or, would it be okay to drill a couple of small holes in the end of the fork leg, and feed in the braze there?

No doubt, there is more than one way. Please share your suggestion.

Edit: going back in the memory machine, I recall that Hank (Henry James) provided instructions about forming rings of brazing material, and inserting them into the ID of the tube before inserting the drop out and brazing. And that's what I did for those. I suspect the same technique could be used here also.

I have done the rings of filler and also have filed half moons on the side of the blades. I have a boroscope to look at the brazing from the inside.

Yes to a ring inside, FAR superior to any other brazing method I know.
Wind rings around a mandrel. Trial-and-error to get the mandrel size just right, due to spring-back. You want the rings just a skosh too big, so they spring down to fit in. That leaves them pressing tightly against the inside wall of the blade, so they don't move around, and best heat transfer = fastest melting once you're up to temperature.

A couple winds of smaller wire is better than a single wind of larger, but I don't know where to get brazing rod smaller than 1/16". That's small enough. Hank Folson supplied "springs" made of really fine wire, and those were great, but I've never made them that way myself.

Just hold them at brazing temperature a bit longer to ensure filler flows everywhere.

You could also ask Richard, tho I expect he'll say the same.

Here's how I make the rings:
by https://www.flickr.com/photos/54319503@N05/, on Flickr

Much like how we did 'em one place I worked. Run the lathe in the back-gear, slowest possible speed, and be very careful when you let go, it'll snap around and hurt you if your finger is in the way.

Richard said he notches the tube, and feeds from there.

When using rings before, it was a little nerve wracking waiting for the heat to melt the braze. Eventually, the braze showed at the gap, but that's not my favorite method of feeding a joint.

He has!

I notch my stay ends on two sides and feed in from there.
I've never used preforms.

That's my favorite part!! Watching the gentle glow slowly darken as filler flows through and then seeing the filler seep out the bottom.