According to Bill :

Scrape the old plaster around the pipe and repatch with patching
plaster. I would then look for one of those split ring type collers
that was suggested. If your handy, you could find a column cap the same
diameter and cut it in half and nail/glue in place. If you want a
perfect seam on ring, buy two. Cut each one just off center so when you
put them together, there is no gap from the saw cut.

Note that it's sometimes possible to duplicate texturing. I've done
it a couple of times, and it's been pretty much invisible.

[Just a temporary thing until we strip all the texturing off. I hate
texturing.]

Experiment with thick mixtures and dab it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.