Canonical is and will not be treated any different than any other contributor. Meaning: Sometimes happens that people forget about patches, while at the same time respond more quickly to a patch from a familiar name. That is just the way things go. But if someone pretty much dumps some code without discussing things beforehand, then likely the maintainer will have a different idea on how things should've been implemented. Which increases the chance that the patch is either ignored (because it is massive) or rejected for needing more work (because maintainer doesn't want it that way).
Easy ways to ensure your patches are accepted: Discuss beforehand, become a "known" person, become a maintainer. Pretty much the same as in any other free software project.
Within GNOME, maintainers have a lot of decision power (which is logical IMO as usually the maintainer will end up supporting the code).
PS: There have been some huge controversies in the past regarding this, 2.x timeframe. At the same time, IIRC there also have various contributions that were accepted outright.