While the guaranteed magic jewelry mods do stack with random mods, they will never exceed the mod cap on Demise.
UOGuide says they can, but I have not yet seen one of these exceed the mod cap on OSI. Testing it is difficult now, because the magic jewelry no longer gets random properties on OSI (instead, people can imbue whatever they want on them now). When you imbue DI on a Turquoise Ring on OSI, by the way, you replace the given DI mod; you cannot go over 25 either!
Before the random mods were removed on OSI, I did run a few preliminary tests with Brilliant Amber Bracelets, and none of them ever exceeded a mod cap.
