Do you realize that the N64 cartriges are not CDs and must be stored in the Dreamcast RAM in order to run? (at least at acceptable speeds)
Do you realize that Dreamcast has only 16MB of main RAM which must be used to load a N64 ROM, to create a virtual N64 RAM (N64 has 4MB RAM, 8 with the expansion) as well as for other "stuff"?
Do you realize that even the fastest existing N64 emulator can hardly run on an x86 CPU that runs at 200MHz and needs at least 32MB RAM?
I don't say that it's impossible to run a N64 emulator on a Dreamcast. But in the best case, you could run games that are small in size (probably 8-12MB) and the speed hit would be big, except if the developer spends a lot of time to push his skills, knowledge and the Dreamcast to their limits.
As for Project64 itself, porting it to the dreamcast is simply impossible (due to the structure and requirements of Project64).
In any case, creating a new emulator designed to run only on a Dreamcast would be the only option.