Is there any way to help live drummers play in sync with live looping players with out requiring the drummer to trigger MIDI beats or listen to a click track?
MIDI drum control surfaces and click tracks are the most common ways to deal with the issue. If those are out of the question, these are some alternatives I can think of.
It is very important that you test any of your choices to see if the dynamic is consistent enough for live performance.
I played in a band with live looping where the guitarist's looper fed a metronome output to the drummers monitor. The metronome would only play when loops were active so at that point the stage volume was usually high enough to drown out the click for the audience (unless the listener was side of stage). I played bass and barely ever heard the click, I just played to the drummer.
That way works fine when the monitoring situation allows but if each player doesn't have a separate monitor mix it's not really viable, unless you can handle hearing the click track through all monitors. We never came across that problem but the solution would've been this:
A silent, vibrating metronome that can sync with a midi signal.
I assume a midi - usb cable will allow it to be controlled by a midi device other than a DAW but I would confirm that before buying..
As far as I know, there really is no other way for a drummer to sync with a looper without an audible click track (or flashing light I guess) asides from just listening to the loop itself obviously.