Just bought a halo spin dr rear hub. The online PDF instructions seem to
imply you grease the ball bearings in the freebub by pouring oil into the
outside of the hub. However, there's a hole in the freehub body where
you can see the ball bearings – should I not just squirt some grease in there? I assume the whole will be reasonably covered when the cassette is in place?

Cheers.

So it turns out that that hole is a grease hole. Squirt some grease in with your grease gun. Job done.