Would anyone have them? Or rather do they exist?
3rd Edition has the portable rocket launcher, which is more of a panzerschreck style weapon, and the mortar rules just don't seem to fit.
Were they ever published in a WD?
Re: 'Official' rules for the hobgoblin/nippon rocket launche
None that I have ever seen. However, in true Oldhammer style you could invent your own, including lots of d10 charts, and publish them here. They would be very popular reading I am sure.
Re: 'Official' rules for the hobgoblin/nippon rocket launche
A simple approach would be to use the rules for the Nippon launchers in Ravening Hordes but give the hobgoblins a slight detriment e.g. a slightly increased chance of misfiring or something. Or assume that Hobgoblins are actually cunning and relatively intelligent for a goblinoids and stick to the same rules as for Nippon .
Indeed it did, there are rules for a generic 'Skyrocket' on page 117 which would do the trick. It's in the Engines of War section under Portable Weapons. What an amazing book!