I’ve been doing this with my recent game in this thread (I actually came on here to make another post there). It’s been pretty good; some times it gets a ton of feedback and sometimes little, depending on time of day, what kind of content, etc. I think it helps to indicate what kind of feedback you want on your post and each sub-post, because for me sometimes I just want to talk and don’t care if no one is listening, but other times I need crucial help fast. So indicating that is good.
If that doesn’t work, messaging people you trust can help, and mockups can too. When I had the idea to make a game called Grooverland honoring Chandler Groover, the first thing I did was make a very basic mockup in Inform and send it to him, asking if it was okay. I’m not sure he even looked at the mockup because he liked the idea either way, but it was nice to have the option.
If you do post a mock-up, it could help to indicate how long it is; people are often leery of starting a game and having it last for hours, so if you say ‘this is five rooms and you can look through them in 15 minutes’ it could draw in more people.
Releasing your game publicly can disqualify you from several events, but publicly shared code does not (at least for IFComp (rule 3) and Spring Thing), so if your mockup is small you could post it all here in a collapsible section, or, if larger, you could just dm people the file. (edit: I decided not to post on my dev diary right after this after all because typing it out fixed the problem for me).