Moved from the web site.
The example here may be one of the exceptions, it reads “Presented to my stepson, F. Cavalier, March 23rd 1899 in recognition of his dutiful perseverance and honourable conduct in my business. W Cockerton”.
Gilt or electro plated surfaces cannot sensibly be engraved.
Engraving Gunmetal is probably not a god idea.
Gold plated watches can be a problem: A Dennison rolled gold (Star Grade) case started life with about 0.036mm of gold on the outer skin and were electro plated on the inside, so after years of wear and polishing the outer skin is probably going to be too thin to take an engraving without going through to the composite core, and an engraving on the inner skin would definitely go thought to the core.
A Dennison Filled gold case started off at 0.072mm of 10 carat (Moon grade) or 14 carat (Sun grade) gold on both sides of the composite. Clearly having double the thickness to begin with means that these cases are more amenable to engraving but remember that 14 carat gold will wear quicker than 10 carat so a Moon case will be the best bet – although still risky, particularly on the back.
I assume, but can’t find a definitive statement to support, that the dust cover was of similar thickness and without pocket wear should be able to take an engraving, this is also the place to do it to minimise the reduction in value caused.
I don’t have information about cases from other manufacturers but guess they would be much the same when from reputable manufacturers.
To sum up, my personal view is do not engrave a watch case! Think about a personalised box or an inscribed watch fob medal to be worn on the chain which could be engraved without endangering the watch.
Finally I am not able to get engraving done and any you have done is at your own risk!