Personally I think this is a great step forward to promoting this site.
Just as @imsodin stated in his comment: "In general I don't think tags need to be introduced formally, new ones should just come up naturally."
The real question here is how to name a new tag for questions that deal with accessibility issues for people with mobility or other issues. The term accessibility, could be wrongly interpreted to mean a large host of meanings, such as parking accessibility or park accessibility limitations due to closures for one reason or another.
To be honest, I am not sure how such a tag should be named, but here is what I might suggest (I am not trying to get the greatest answer here, but hoping to see this site genuinely expanding for those with real life physical limitations):
Personally, I believe the tag should be disability-options. The tag should be named in such a fashion that the average person would understand its' meaning and will not give offense to other with physical limitations. I think this tag name would fit the bill. My wife has serious physical limitations and has told me that the term "disability-options" is not offensive to her in the least. If you guys think otherwise, I am good with that too.
In any case, cobaltduck, I wish to thank you for bringing this subject up, this not only will make many individuals happier in the Great Outdoors, it will expand the awesomeness of this site and communicating that we are thinking of others.