[O] Restricting admisible tags depending on header

[Óscar Fuentes]

I'll like to offer only certain tags depending on the contents of the
current header. For instance, if the header is

* Status

I want to see only "open" and "closed" on the options shown by
org-fast-tag-selection.

Currently I define quite a lot of tags with #+TAGS:, most of them are
intended to be assigned to a specific header. Having so many tags
displayed by org-fast-tag-selection is confusing, error-prone and breaks
the selection method (org-fast-tag-selection runs out of letters).

I'm thinking on advising org-fast-tag-selection and prune the list of
acceptable tags before calling the real function, but maybe there is a
better method.