I'm going to echo SJS's sentiment here. If a stamp like that is introduced, it'll have to be used by the whole userbase, and they'll all need to learn exactly what does and doesn't work properly in P:M.
Unless the PMBR decides to release an exhaustive list of what kinds of mods will and won't work with their fancy codeset, knowing what works basically amounts to testing it on P:M directly. So to know what requires the marker, you'd most likely need to be using P:M yourself.
There are definitely more content creators on the Vault than there are P:M users downloading from it. We're talking about a feature where a majority has to set up Project M and test their hacks, so that a minority doesn't have to. That sounds incredibly inefficient to me.
It would then be up to the previous hack uploaders themselves to determine if they want to go in and update their posts or not, but at least the new guys can come in and place their hack in whatever category since it will already be built into the uploading interface
That is also a problem. Compared to say, the Wi-fi marker, which was introduced early in the BV lifecycle, a P:M marker would be missing from
4 years of hacks.
In essence, that would means
years before the marker would start holding any real significance on a global scale.