Is it acceptable practice to write a fiction novel set in an imaginary universe in first person plus have the main character be an alter ego of yourself? (The main character has my voice, essentially.) Are there any precedents for this?
It depends entirely on whether your voice is an interesting one. We all tend to believe our own voices are more interesting than they really are. Despite all the talk about expressing oneself, what readers really want is something interesting to them, and there is perhaps a better chance that you will find something interesting to write about out in the world rather than between your ears.
But that said, all fiction is really in the author's voice -- in the author's words. To say that any other voice is speaking is a conceit. However outward looking the author may be, it is still their telling of what they saw and how the interpreted it.
So in the end this issue is probably not your voice, but what part of your thoughts and observations that voice expresses. If it is inward-looking and self indulgent, most readers will probably not be interested. If it is outward looking and honest they may well be.
Autobiographical fantasy is not unheard of --Borges practiced it in many of his short stories, and the Baron Munchausen novel was based on a real life autobiographical fabulist. The frame story for The Princess Bride is fictional autobiography.
In practice, for the reader, there's no real difference from any other first person novel, except in as much as you want the reader to actually believe these events happened to you.
As a writer, the problem to avoid when writing an alter-ego is that it plays out as a shallow fantasy of who you would like to be, or what you would like your life to be --such narratives are generally uninteresting to other people.