Maybe I'm missing something, but isn't the offset parameter just another
way of spelling
buffer(input, offset)?

I like the avoiding of copying, just wondering if having a magic
parameter to get a tuple is really better than (say)
result, used = zlib.decompress2(source)
if used<len(source):
    result2, used = zlib.decompress2(buffer(source, used))

This changes two things. Rather than a magic parameter it adds a new
function which always returns tuples (meaning that you don't need magic
to make sure users don't accidentally pass offset=None and get a single
result when they wanted a tuple), and uses the built in buffer facility
to avoid copying rather than relying on delayed slicing.