Family legend has it that my great-grandfather was pressed into service by the German Empire on the Eastern Front at some point during 1915-1917. He and a few thousand others were sent to rush the trenches with clubs and other crude weapons as "cannon fodder." I was not able to find any credible sources on this topic online.
Is there any merit to this story, and if so, what battle would this have been at?
There is definite merit to the story, although pinning down an exact battle with such little information might prove impossible.
Before 1918 there wasn't a true independent Poland, and somewhere between two to three million Poles were conscripted by German, Russian, or Austrian armies during the first world war and often sent to fight each other. The specifics of them being armed with clubs is likely just a story being escalated over the years for dramatic effect, but foreign conscripts are often poorly equipped and treated as expendable.
After 1917 many of the former conscripts ended up joining the Blue Army or other, lesser known units to secure Polish independence and fight in the Polish-Ukranian war. Their battlefield knowledge and discipline would help form the backbone of Poland's emerging armed forces.
Much of this is covered in The Cambridge History of Poland, if you wish to do further research, but I am sure that someone who specializes in the area will come along with a more knowledgeable answer than mine in a few hours. Perhaps /u/kaisermatias or /u/Caherdaniel would know more.