This resulted in hair drawing with an offset if an instance_offset was
set.

note: Usually the instance_offset gets combined with the objects obmat
in 'make_duplis_collection' / 'make_dupli', see

/* Combine collection offset and obmat. */

Using the resulting DupliObject->mat instead does include the
instance_offset, but the results in double-transforms (something that I
have not investigated further -- but might be the better solution?)