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.
Having good monitoring conditions often reduce the need of a click track, since the loop and other sounds work as a click track would. The drummer just needs to follow it as it was any other instrument.
You might want to try some kind of visual help, like a visual metronome where beats are displayed as numbers and/or colors and/or intermittent flashes of light. You can also try sensitive aid, like vibrating metronomes.
If the looper has speed controls, another alternative is to manually (or footually or whatever) make the loop sync to the drummer. Basically, the one in charge of the loop is using the drums as metronome. This needs experience in beat sync (and a good pitch algorithm if it's not a MIDI-based loop), so it is most often than not more trouble than it's worth.
Also, your platform might have a way to make the looper sync to the drummer. Your platform might have tempo tracking. It's worth to check it out, and even maybe move to a platform that supports it. Ableton Live has a Max for Live plugin called B-Keeper that syncs Ableton Live to the drummer. Even if you don't use Ableton Live as the looper, you can use it as the metronome controlling your loopers, as the master MIDI clock. You can then make everything sync to Ableton Live, which is being tempo controlled by your drummer.
Another example of tempo tracking is Circular Logic's In Time. It tracks the drummer's (or other musician's) tempo and uses it as the master tempo of the sequencer/looper/gear. Here is a tutorial on how to use it with acoustic drums.
If you don't want to use MIDI control surfaces, an alternative that might work for you is to use audio to MIDI conversion. One piece of the drum kit serves also as a MIDI trigger, like a MIDI drum control surface would.
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.
Other than the hardware control interface, the BodyBeat Sync’s metronome can be driven by two additional methods with the help of the USB port located on the right side of the unit. Connect the BodyBeat Sync to a computer using the provided mini USB cable. Select USB Audio Device as the MIDI output port to transmit a MIDI clock signal. W hen this signal is present, the tempo setting on your BodyBeat Sync is under the control of your MIDI capable software.""
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.