Pardon my naive follow-up question: if I put the required header files into the correct include-dirs, inkscape will build without linking to any libboost_*.a or libboost_*.dylib?
Yes, it should build fine as long as you have the required headers in $prefix/include/boost.
The files installed by MacPorts into ${prefix}include/boost/ aren't few either:
That's because they contain Doxygen documentation as well as the code. If you are concerned about space, I think you can trade some compilation time by putting the boost directory in a zip archive and then mounting the archive at /usr/include/boost using MacFUSE. On Ubuntu I would use the archive mounter available in Nautilus and symlink the directory created in ~/.gvfs to /usr/include/boost. The size of Inkscape binary won't be affected by the size of Boost headers, because the header-only libraries are templates that do not generate any code if they are not used.
Regards, Krzysztof