said by Zach1:
For the current monitoring portion, I'm thinking two standard diodes in forward bias between the '+' and '-' of the bridge output. This configuration would give you 1.2~1.4V across the diode array when there is a load on the circuit.
That's a really bad idea when dealing with separate circuits. There should be galvanic separation between the uC circuit and the doorbell circuit. An optocoupler or relay should be used in such an instance. Imagine the transformer fails and you end up with high voltage on the 24V line... Not to mention you are starting with a floating circuit (24V) so you don't have a defined potential in respect to the ground or to the uC circuit.