I used to make my own “semi-rocket” build in 4.12, basically I use “-precompile” flag to tell UBT to make precompiled libraries, and wrtie a bat file to collect files into my own “rocket” build folder (by comparing files difference between source build and launcher build). Not sure will apply to your situation ( I did not build build server target )…
Btw, how is FastBuild UE4 integration performance? Will it help on signle machine build?