Since External Instrument cannot adress the Main/Master outputs you are either
one mono output short or have to use workarounds like the one suggested by Nogi if you want to use panning tricks. Most of these workaround will end up with at least one mono channel of the Main/Master output being outside the Drumrack folding. There are some solutions that can around this, but they are rather inconvinient. It may be better to use only 15 pads instead with the Main/Master outputting only one pad.
Strangely it seems as if NI Kore cannot be used for these kind of things at all. External Instrument doesn't offer Kore as a viable source/target to chose from.
1. Use (a Rewire capable DAW like) Reaper, load Guru in Reaper and control Reaper via External Instrument over Rewire. Reaper is able to access all outputs of Guru either as stereo pairs (even in uneven combinations like 2/3 instead of 1/2) or mono channels. You can access all stereo/mono outputs of Reaper from External Instruments as it seems (which suggests that the limitations with Battery and Guru are from within the plugin rather than Live).
Create a single track in Reaper and change its number of channels from the default 2 to 16. Make sure Midi Input of that track is receiving from Live. Load Guru on that track and route its panned outputs through the mono outputs of Reaper. Record arm the track and turn on monitoring.
2. Use virtual audio ports or rerouted physical ports in order to use Live's track rerouting to split the main outputs. I don't even dare to think of a real plan how to set this up, it's just too awkward.
PS: There seems to be some problem with the combination of Live/Rewire/Reaper which I will report as a possible bug. Live reports upto 6999% CPU load upon loading Reaper while real CPU load is about 0%.
