I actually care only a more efficient linking process, I don't plan to use the Watcom C/C++ compiler itself, but only the linker and library manager.
Compile the source to a object file,
g++ -c hello.cpp -ohello.o
Then link into a execuable using wlink.
Unfortunately, I don't have enough time and resource to try this "compile by gcc and link by watcom" approach now. Maybe half a year later, I'll have a try.
If this approach fails, I think I can port the Windows-specific code in wlink and wlib to GNU ld.
Another significant work I know is the porting of the whole Watcom suite (commandline only so far) to Linux platform by the Open Watcom team (this team have also ported win32api from MinGW to their suite) , it does support mix-linking gcc-compiled objects and watcom-compiled objects.