I'd like to hold off until we decide the larger issue of the purpose of query parameters in the spec.

I do think either way it should be possible to limit on authority, but I'd prefer to see it as part of an overhaul of querying on Actor. That is:
* drop Actor, Object, and instructor query parameters
* add an "agent" query parameter, by default catches records where that agent is "I" or "this" or part of an ad-hoc team which is "I" or "this"
* add an "interpret_agent_broadly" parameter, in which case statements where specified agent is used anywhere (including as an authority or instructor)
* have a similar parameter pair for "activity"

Again though, my suggestion above might make no sense at all depending on what we decide the purpose of query parameters is.