Why do you want to make use of invalid gmail for forum? Does it even make any sense? The best thing which I will recommend is just to have a separate gmail for the forum, or make use of the gmail which you don’t use anyhow.

If you make use of an invalid gmail, what if anything happens to your forum account, and you want to change your password, how are you going to do that? I don’t know if bitcointalk is illegal in any country, but I don’t really think so, if bitcointalk is not illegal in your country, then why are you going to make use of invalid gmail? Are you sacred or what? I don’t know maybe am the only one that do always protect my forum account, so why will I even think of making use of invalid gmail.