Seems we don't have one (weird, I was sure we did).
It should be trivial to implement though.
Can you try adding a new function into adagios/status/rest.py with something like the following:
def get_downtimes(request, **kwargs):
l = adagios.status.utils.livestatus(request)
args = pynag.Utils.grep_to_livestatus(**kwargs)
return l.query('GET downtimes', *args)