I think it is possible (and easy) to implement with
Data.ByteString.Builder. Just use the low-level interface: http://hacka
ge.haskell.org/package/bytestring-0.10.8.1/docs/Data-ByteString-
Builder-Extra.html#v:runBuilder

BufferWriter takes a buffer to write to and returns a continuation to
be called in case the buffer is too small. The idea is to allocate a
buffer, but reserve necessary space for length and checksum. Then run
the builder, get the resulting ByteString (or work directly with `Ptr
Word8`), calculate length and check sum and write then into the
reserved space.

Just be careful when using the low level API, and read the docs
carefully.