My posts are 100 x 75mm and my rails are 70 x 45mm. What would be the best way to notch out the posts so the rails are flush with the outer face of the posts? I can't notch out the posts the full width of the rails as the posts are only 75mm thick. Could I cut the end of the rails at a 45 degree angle and cut an angled notch in the posts?
It would take a lot of work to get a clean notch on the posts but it would work. If you are not concerned about the look of the post itself, you could rip the corners of the posts off with a saw at a 45 degree angle with the face of the cut the same width as the face of the cut as the rails, then toe screw or toe nail them in place. It would not be as strong as notching each one in, but it will be much faster. It would be a stronger attachment than just cutting them between the posts. but that is an option as well.
This is pretty ambitious, I'd be tempted to give it a try but might be kicking myself later. I could see a couple possible problems.
Furniture sits on a nice flat floor, and houses sit on nice stable foundations, but fences have to follow the contour of the ground. Unless you're building your fence on the Bonneville Salt Flats you have to fit your fence to the ups and downs of the land. Making the joints to follow the ups and downs will be a challenge.
Over time, the ground will want to move some and tend to heave the fence posts around. Of course nice deep postholes will help keep this to a minimum but there's still likely to be some movement. That will be a challenge too, the joints will have to be strong enough to hold together against that movement or have some flexibility to move over time.
Realistically, I think between the two, fence rail brackets sound pretty good - much less work and much easier to repair / maintain.