Not the literal flag, but a little sign e.g. Halachic Authority Must Be Consulted, Thank You Moderators? Any symbol will suffice.
As far as it concerns site members and specifically, the question asker, I don't know if that's necessary. I think the "halacha" tag wiki and the FAQ aught to be sufficient.
As far as it concerns one-time or occasional visitors, there already is a box on the right sidebar (visible when you're not logged in) that has the same stock disclaimer as the FAQ. I think that's enough.
Adding a specific feature for this site is unlikely to happen, and adding the disclaimer to the bottom of every halacha question (859 of them), is impractical and will just clutter up the question body.
SO has (paid-for) symbols for various tags (the flash tag has an Adobe logo, for example). Perhaps we can get, for our halacha, health-safety-shmira, and psych-mentalhealth tags, a symbol like this one. (I'm mentioning this not because I think it's necessary but because I think it's sufficient, and perhaps those who want a notice will think so, too.)