also I cannot really get to add keys to global-meta, the use case being having all the tags for all the posts accessible from any task

so you want, to dynamically add keys to global meta. That makes sense. I think originally that was planned for static data, but I see no reasons why it should work with dynamic data too

yeah, I worked around that by adding things to the :entry key inside perun/assortment but some things like all the tags available for all the pages should be global