TLDR: FTP Master rejected my libpaper package because it contains gnulib source files. I pointed out that other Debian packages for which I am upstream do exactly this and have been accepted, and that it is the standard way to use gnulib. A few senior Debian Developers said they did not consider this use of gnulib to be against Debian policy. But FTP Master's stance appears to be that they will not let any new packages into the archive that contain gnulib sources (or in general, vendored sources—they don't have anything against gnulib in particular!). I also argued that building against Debian's version of gnulib would risk introducing bugs (I have found that updating gnulib in my projects can make previously-working code fail).
It's Debian-specific, though I imagine other distros might also take a similar stance.
In this case, the solution is for someone else to repackage libpaper without the offending files (by generating a new source tarball). I have said I don't want to do this myself; to be honest it's just a depressing thought to spend hours doing something that makes no sense to me, and that will potentially cause me bug reports in future.
I do sympathise with Debian's aim here, and the long-mooted "libposix" project, or rather an extended "libgnu" version—that is, an installable version of gnulib that one can use like any other library—would solve this problem for both me and Debian. Maybe I'll summon the energy to tackle some of the libposix to-do list one day.
Sure, I'm just complaining, not asking for a solution. I should have been clearer about that, sorry.