You will find just a few Huddarts in the IGI and all seem to be from the same area of Cumberland in the north west of England. There is also a lot of information on the East India Company on the internet.
If you want a copy of the photo of the gravestone, just let me have your contact details. There was a small booklet which gave details of the Old Protestant cemetery in Macao but, interestingly, those who had died and had been buried elsewhere initially, including your William Huddart are not listed among the tombstones [there were about a dozen gravestones which I noted, none of which were listed but all were from before the cemetery was 'announced' so I guess the people had been recently buried outside the walls of the city [the Portugese did not initially allow Protestants to be buried inside the city walls].
Happy to help out if there is something you might need from this side of the pond