Good Morning Yair!
A 404 is a 404 plain and simple. And if Google was able to report it, then they were able to get to it. Which basically means, someone else could. I'm not sure what platform you are using currently using but there are plenty of easy options for a quick fix.
If you wanted a simple option you could just throw up a 302 if you are planning on putting content up SOON. Temporary redirects differ from 301 because they are temporary. Google has to evaluate if they should keep the old page, so in this case, when you are planning on launching the old one (well, technically new one) you might be able to use that to your advantage.
A better idea would be to noindex/follow the pages (before they have content) and let the links to these pages be follow links.
I was unsure exactly what you meant by this. The only difference between follow and no follow links is that you are telling Google not to let your linkjuice carry over to the page you are linking to. You are linking to the page without vetting them. My apologies if you already knew this, but I was slightly confused by your sentence. Google will still go to that page to check it out.
Another option: change your schedule and don't put up pages that far in advance that you aren't planning on publishing. I edit everything offline. Google likes new stuff, especially content. Of course they take longevity into account as well, but as far as making a big splash, putting a website up piecemeal is like having people show up at your birthday saying "Suprise!" when you answer the door when you were the one who invited them. The notion is nice, but it just doesn't have the same effect. Not only in the Google realm, but even more so in Social Media.
My favorite option, and my personal recommendation would be to play the cards you were dealt. Get rid of the 404 and the 200 and embrace that you have a new page! Go on and write a profile piece for the member. Write some sort of Biographical data that can act as a placeholder. At this point it doesn't even need to be stuffed with keywords and amazing seo phrases.
On Second Thought: I'm not sure exactly what your forum is for, and if this issue is specific to a few members or if you are referring to bulk membership, but I have a few ideas on how you might be able to extrapolate some of the signup data into even a simple post to avoid getting a 404! Even if you parsed some of the forms from signup and created a simple little one page that displayed, I think that could help.
At the end of the day, Google loves forums that are strong and authoritative. They also understand that every single person on a forum isn't going to post and isn't going to interact. So depending on what kind of forum you have, and what exactly you are doing, some of the forum issues will just have to be accepted. I think it would be more valuable to clean up negative linking and and analyze internal issues and build form recognition etc. than fix soft 404's coming from a handful of users. Again all of this depends on the size of your pool.
Also, you could just track their usage, if someone is logging in all the time and not posting then fine, I would deal with it because who knows what they are doing in the real world. If someone made an account in 1999, and then threw their computer out of the window in Y2K and never bought another one because they still believe that everything crashed, well then maybe its time to fix that 404.
Hope that helps!
Sorry if my train of thought is a little off this morning.... not enough coffee!