I have found some physical proof: here is a photo of a board of dials and their associated references from the Heuer factory archives. The dials show a Carrera style dial with Zodiac printed on it (top right); Heuer have even given the dial it's own reference. The dial at top right corner is easy to see, I think the bottom row also has a Zodiac dial, but it is a bit blurred so I'm not sure - maybe some one has an in focus photo.

In case anyone needs proof about the board, thirty or forty other people can vouch that these dial sheets exist and are from the Heuer factory. Probably 20-30 have their own photos of the sheets, so can verify that this is not just a nice piece of photoshop work (albeit would be easy top do). I hope this is sufficient.

On these board sheets there are also numerous examples of Abercrombie and Fitch dials, I don't think anyone questions whether Heuer made watches for A&F (there are A&F watches in the Heuer museum). From there you have proof Heuer did make watches for other companies (admittedly not a Swiss watch manufacturer). There are also a lot of dial with no Heuer shield and a blank space, presumably to allow different names to be printed onto the dial.

I have one more example which no one mentioned, as it is pretty obscure; the first execution Autavia made by Heuer for Kingon. Here the Heuer trademarked name (Autavia) is used even though the Heuer name is replaced by "Kingon" (on dial and back case). The watch is otherwise identical to the related Heuer. You can read about it in a good posting by Eric Wind on Hodinkee.