Take a look at Question About Passing Environment Variables for PBS Job and the related https://github.com/puneet336/OOD-1.5_wrappers/tree/master/openondemand/1.5/wrappers/slurm/bin
The idea is you have wrapper scripts on your x86 machine that is running OnDemand and instead of installing the Slurm client binaries on that machine you use ssh wrapper scripts to execute the Slurm binaries on another host (a login node?) of the ppc64le cluster. Of course you would need to setup trust between the two so you could ssh without requiring authentication. Perhaps this would work.