The Zimmerman thing came later--after the US joined the war, remember this was 1915--but there were some ominous foreshadowings of the laterGerman attempt to influence the Mexican government against us. It's clear that the Germans wanted to keep the US busy at home on its own borders and may have had some dealings with the insurrectos.
Most Notable is the fact that in addition to vowing genocide against Anglos, the manifesto specifically decreed that Germans and Japanese would be exempt from death.
The Japanese link is very curious because the WWII axis between Germany and Japan was not even on the horizon at this point. In fact, identified among the dead raiders at one point were some ethnic Japanese!! The Japanese had a small fleet on the Mexican west coast on a 'good will' mission at this time. No one knows much about this item and there are very few documents existing. Perhaps someone will come across something in Japan one of these days to enlighten us. It may have been part of a future-looking strategy by the Imperial staff.
Well, as you know Japanese policy always looked decades ahead. At the time Japan was one of the Allies, but always had a trans-Pacific interest. For them to have a view of mischief in Mexico is not surprising, and you should pursue this line of reasoning.
As for the Germans, I always felt Germany had an interest because so many Texans (and nothern Mexicans) were of German descent and that it predated Zimmerman. When reading about the Zimmerman controversy, I was left the impression this was a culmination of German diplomatic efforts, rather than a desperate ploy.