[lxml] Is there an ElementTree class lookup hook?

lxml provides support for custom Element classes (as well as element-ish e.g. Comment or PI) via the `ElementDefaultClassLookup` registry, and the ability to hook it into a parser. But that registry does not seem to have a slot for the root tree of the elements. Is there a hook somewhere to set *that*? I tried looking around the API docs but nothing really jumped out. PS: the documentation for `set_default_parser` explains that it sets the default parser *for the current thread* and that "You can create a separate parser for each thread explicitly or use a parser pool.", does it mean that in a "don't call any API which gets an implicit parser and manage your parsers by hand" sense or something else?