Our distribution is based on .dproj files that are compiled by MSBUILD and this has been working for years and we had not a single other request for customizing packages or .cbproj based packages. Also, for us, this effort and extra complexity needs to be multiplied by 16 C++Builder versions we support. You understand that this isn't feasible.
For your package compile issue, I suspect the correct TMSDEFS.INC file is not being picked up and suggest to look at this first.