Think it through — what does inline do? It creates a C/C++ function for you, then compiles and links it into a dynamically-loadable shared library. Where does that one sit? In R’s temp directory.
So you tried the right thing by shipping the R frontend calling that shared library to the other process (which has another temp directory !!), but that does not get the dll / so file there.
Hence the advice is to create a local package, install it and have both snow processes load and call it.
(And as always: better quality answers may be had on the rcpp-devel list which is read by more Rcpp constributors than SO is.)