Don’t know of examples, but here’s what I’ve construed and turned up. Maybe it’ll help point you in the right direction.
The comment before the Tools.UCB.ShowHelperDialog()
code and the line HelpID = aEvent.Source.Model.Tag
together suggest the tag property associated with the event triggered by pressing a button on a dialog needs to be set. Doing so may require creating a custom event handler, but the comment seems to suggest the button itself should have a tag property which would presumably be associated with the event. (But maybe it’s just a badly worded comment…)
How OpenOffice.org Help Works - The Document Foundation Wiki mentions use of Help Ids in how Help works.
Finally, there should files ending with .ht (e.g. swriter.ht, scalc.ht, etc.) in your LibreOffice installation help; they seem to include HelpID definitions. You may have to poke around to find them. On my system, they’re at /usr/lib64/libreoffice/help/en-US.