If you need to debug something in the early ld.so startup, and can't refrain from good old printf debugging, there is a caveat: the available API in ld.so is rather limited. See the few functions is dl-sysdep.c. For example, there's a private __libc_write, which you should be able to use for writing to FD stderr -- but, at early ld.so startup, this isn't usable as _hurd_init_dtable is still all zeros, etc. To get you started, here is a simple dl-sysdep.c.patch to get access to the Mach console. Original submission.

Can this be integrated with the other debugging printf functions from elf/dl-misc.c (_dl_debug_vdprintf) (open issue glibc)?

In 2013-01, the mach print system call has been added to GNU Mach. It is available when the kernel is built with the in-kernel kdb debugger.