Some SE sites, such as Math, have at least one "sandbox" type post on their site's meta (e.g., Sandbox for drafts of long, complex posts for Math), with them normally being used for users creating drafts of long posts, checking on formatting, etc., before posting on the main site. This is not quite the same thing as what you're asking about, but I believe using one of the answer slots on a post like that would work fairly well for what you're describing. In particular, with it being updating a current "answer" on the post, relatively few people would notice it being updated plus, with it being a "sandbox" post, those people will often just ignore it anyway. Also, as fixer1234's comment indicates, if the score is low enough (i.e., -8 as Laurel's comment states, with this being used in the Physics' meta sandbox post), then edits won't bump the post to the main page. Nonetheless, the post would not be completely hidden, so people could still view the incomplete post. Regardless, if something like was used, I suggest adding some sort ofa prefix text to indicate this is what's happening so people are aware of it and nobody else would try to edit that answer in the meantime.
If what you're doing would involve many edits, then note that after the 10th revision, an automatic flag is raised. A small advantage of using a post dedicated to having its answer slots being used a lot is that moderators will likely not pay any attention when any of its answers are edited many times.
If the site's meta doesn't already have a "sandbox" type post that you may reasonably use for your purpose, then this may provide a relatively good reason (among possibly several other reasons) to create one. Note that, as Laurel's comment indicates, there's a list of sandbox posts, with this including that sandbox type posts may be used for various purposes, and there even may be more than one on a particular site (e.g., Math and Meta), with each one being used only for specific purposes. Alternatively, you can create a new post on the site's meta to be used just for doing what you're describing, although it may be best to first check on the site's meta to confirm this is acceptable.