Basement slab - 1 toilet
First floor - 1 full bath and kitchen
Second floor - 2 full bath
Third floor - 1 full bath with 2 sinks and both a shower and tub
The stack vent is on the back side of the house, a 3 story addition was added to the back to add bathrooms (120 year old house). The connection between these add on bathrooms and the main stack is a maze of pipes because they tried to avoid punching through the old exterior wall to get to the main stack. The second floor drains into the first floor which makes a horizontal you shape to go through an old basement window that is 6' away from the main stack. Needless to say it doesn't work very well.
Since all 4 bathrooms are in the addition it would be a lot easier to route the main stack through the addition (or add a second stack that Y's off of the main stack) then punch several holes through the 18" of brick for each bathroom on each floor.
I think I am a bit mixed up on terminology. When I said 90* bends I am thinking of those long sweeping 90* bends, not the hard sharp angled ones. Is this what you mean by sweeps?
By dry vent do you mean that the fixtures need to be vented separately from the main stack?