Fairly sure they have lightened up on bit regulation quite a bit this past year. From what I can decipher from the new rules there is no rule about different metals anymore... ?? I hate how the USEF writes their rules :P The best way to know is to call a steward in your area.
"All bits (in A and B below) must be smooth and with a solid surface. Twisted, wire and roller
Bits are prohibited. A bushing or coupling is permitted as the center link in a double jointed
Snaffle, however, the surface of the center piece must be solid with no moveable parts.
The mouthpiece of a snaffle may be shaped in a slight curve, but ported snaffles are prohibited.
A bridoon is defined as a snaffle bit used together with a curb bit to form a double
Bridle. Bits (including curb and/or bridoon bits of a double bridle) must be made of
Metal or rigid plastic and may be covered with rubber (in manufactured state); flexible
Rubber bits are not permitted, except as noted below, under A. The diameter of the snaffle
Or bridoon mouthpiece must be minimum 10 mm diameter at rings or cheeks of the
Mouthpiece (exception: for ponies, the diameter may be less than 10 mm). The diameter
Of the curb mouthpiece must be minimum 12 mm at the cheeks of the mouthpiece.
Snaffles used in Young Horse classes must have a minimum diameter of 14 mm. Any bit
Combining any mouthpiece pictured in Figure 1A with any cheekpiece pictured in Figure
1A is permitted. Type of bit should not vary from those pictured below except where specified,
And bits should be attached only as pictured in diagram. NOTE: FEI Level riders may
Warm up only in a double bridle (with both bit and/or bridoon made of metal or rigid plastic)
Or metal or rigid plastic snaffles pictured under B. In both cases, bits may be covered
With rubber (in manufactured state) and and flexible rubber bits are not permitted. A cavesson,
Dropped, crossed or flash noseband is allowed when a snaffle bridle is used in warmup
Or competition, except as prohibited for some tests."
They say money doesn't buy happiness -- well happiness doesn't buy horses!