Dummy answer and one in progress for the same question allowed?
I've seen answers with "in progress" or "under construction" once in a while here in Space Exploration SE, and probably used that once or twice myself. However, in all cases I've seen, it's been done to keep people for leaving comments that ask questions that are going to be answered by the rest of the answer once it's finished being typed and linked.
I don't think I've ever seen it done to "reserve" a space or place for an answer, or to discourage other people from answering here in Space Exploration SE. This is a lower-speed site and quite collegial.
In a fast-paced site like Stack Overflow, I do see answers pop up quickly with what looks like a placeholder answer the way you'd dump a jacket or backpack on a seat before you're ready to sit, in order to make sure nobody else sits there. That's the nature of the
beast community and its more competitive edginess.
As far as "allowed" is concerned, I don't know of any restrictions against this if done carefully. The way Stack Exchange operates has evolved over millions of interactions and almost all of the really bad behaviors have been excluded or made difficult, and voting takes care of most of the rest.
If you are answer-squatting to any great degree, someone will notice and either leave a friendly and helpful comment (hopefully) or flat it for moderator attention, and all that follows.
I've just down voted a placeholder answer to illustrate my point about voting being effective way to address bad behaviors.
This is an absolutely non-answer. It's not a half-written or partially complete answer. "I heard that..." is a comment.