I'm not going to get into it unless I have to, but you don't -really- want a bridge material to be 'resonant' per se. You want it to be rigid and efficient in trasmitting waves of vibration to the body; which is what you actually want resonance in.
It's not a matter of the bridge actually 'resonating' that's important. It's the effeciency in which it transmits those vibrations to more important things like the wood of the body/neck/fingerboard and the pickups.
This is why people always assume that denser (ie: heavier) materials used for bridges are better; thinking heavier means "more solid". Cheap die cast pot metal bridges have lots of air pockets in them (cut an old Ibanez trem and half then cut some no name trem off of a real low end guitar in half and you'll see what I mean).
What you want is an object with good mass and a dense material as void of air pockets as possible so the transmission of vibration to the body is as effecient as possible. Any loss of intensity or vibration can result in less volume, 'holes' in certain frequencies and and overall lack of tone in your intrument (or at the very least it not living up to it's potential).
Brass was always used for this for a number of reasons.
a) it's fairly dense (heavy)
b) it's easy to forge/melt and inexpensive
c) their are many grades of it available due to the mass percentage of woodwind instruments made of it (horns, trumpets saxophones etc)
d) and it readily accepts a variety of finishes from electrochemical (chroming of various shades), powder coating and just plain paint
Hope this sheds a little light on the subject.
Oh damn, I guess I did get into it.