With the later models (Proline 800 onwards) you get some storage back in the cupboard opposite the bog thanks to moving some avionics items out of there.
The other external baggage option is the pannier in lieu of the ventral tank, I don't really know what real world experiences have been but it doesn't look a very practical shape. It was also an option on the 800A&B but not very popular
AFAIK. Personally I'd rather have the fuel (or rather the option thereof)
To be honest I've often been amazed with a bit of practice and ingenuity what you can make go missing in the available space in the Hawker, Last summer we made the entire contents of an Audi Q7 (seats folded and cubed-out full) fit; Nothing was visible except for 2 briefcases. It's generally easier if the pax/crew are educated to use soft bags.