As long as they are on top of the ground rather than dug in, I think they will be great for a garden.
Moose are browsers. The poop looks like sawdust if you break one open. But as browsers they eat the twigs, leaves, etc. So what you're going to get is basically the same as if you took a bunch of twigs, etc and put them on your ground, except they've been through a gut and are already partially broken down. The advantage of this is that the 'little branches' they eat are actually fairly low in
carbon and pretty rich in all of the good stuff, as well as partially digested, as compared with the kind of sawdust you would get from a sawmill. Even so, I think they would be better on top of the ground as a fertilizer rather than dug in (they may take up the nitrogen to break down the still relatively high carbon content).