SP: tagging all entities in MetadataProvider

[Peter Schober]

I'd like to dynamically add an EntityAttribute to all the entities
(well, all the IDPs, but SPs can be filtered out with an
EntityRoleWhiteList before that) from a MetadataProvider.

But the documentation
https://wiki.shibboleth.net/confluence/display/SP3/EntityAttributes
states it needs "one or more" Entity elements (where enumerating all
the entities I'd like to apply this to isn't feasible) and does not
mention wildcards or regexes as potential values of such an Entity
element, either.

So how would I add EntityAttributes to an arbitrary number of entities
loaded from a given MetadataProvider?

Cheers,
-peter