Some of the OP's requirements are dumb including:
1) Non recessed bolt mount <- really, it's one drill bit or alternative sheldon method. Closes off so many options it's ********.
2) Compatible with flat bar levers, really that means looking for a flat bar lever that's compatible with cantilevers. There's plenty of them.
3) Long reach, how about a NUMBER, 27" to 700c doesn't tell us anything definitive
4) Clearance for wide tyres <- moot
Reducing the force required to actuate the brake is easy. Bend the springs on the old single pivots, up and it'll reduce the spring back force. If the lever incorporates a return spring, remove it. Looking for a new dual pivot brake is not the answer. Is the OP also aware that some flat bar levers have reach adjustment built in? That will also help.