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Umberto Eco, E. T. A. Hoffmann and others have pretended (in the introduction to the story) to have found and edited obscure old manuscripts. In my view this added to the charm of their writings.
If I attempt such a thing and do it well, do I risk the publisher denying my authorship and rights?
Pretending to "have found and edited an obscure manuscript" is quite a common literary device. A few other examples include Neil Gaiman's The Dream Hunters (Illustrated by Yoshitako Amano, part of the Sandman series) and William Goldman's The Princess Bride.
You will note that in all examples, while the pretence is maintained within the body of the text, the book is credited to the real author. It's right there, on the cover. Which is to say, it is a game played inside the story, but never crossing over into the world of publishing. You approach a publisher, you present your work as what it really is - your work, using a fun literary device.
You approach a publisher What if its through something like Amazon Kindle, where everything is automated and you don't actually approach anybody?
I don't think so.
And an old one, at that. Some classical, widely recognized authors have used it in the past (Manzoni's Promessi Sposi - or The Betrothed comes to my mind. Another one is Cervantes' Don Quixote). Authors used to do this for a variety of reasons - for example, Manzoni did it so he could place his work in the past and avoid censorship for being critical of the government of northern Italy.
You may find other - more recent - examples in this researchgate question or searching "found manuscript" device on Google.
Aside from being a well-established literary device, you won't risk your publisher denying you anything. They would need a copy of the "found" manuscript to claim ownership, and such copies don't exist, and you can (I suppose) easily prove authorship of anything you've written.
If your work is good - so good that they want to claim it - they will have better chances working with you rather than trying to put up such a scam.
Update and Edit: My previous statement about classical Greek and Latin authors using this device remains unproved, so I have ruled it out from the answer. At the present time, I could only find information about Dictys Cretensis, Ephemeris belli Troja as a possible example, even if it could be a bordeline example between literary forgery and pseudoepigrapha.
I have found other notable examples:
This is an accepted literary device, as others have said.
However, it would be wise to avoid claiming your "old manuscript" was by a historical figure, especially if they wrote books that are still read. That's really asking for confusion to break out. Claiming that the old manuscript was by a living person risks all sorts of complications, and should definitely be avoided.
