Ant,
Wilwood now has a hydo-mech rear caliper as you are describing. They call it the "Combination Parking Brake Caliper"
Here is a link:
http://www.wilwood.com/Products/001-...-cpb/index.asp
The only problem I have found with this type of rear caliper is its lack of mass and surface area. In a Hi-perf/track application, you must run a lot of front bias in the system to keep the rears from boiling out the fluid.
This is a common problem on early Vipers. When the owner does a pad-line upgrade (which they desperately need) The rears get hot so fast that they boil the fluid, burn the seals. Then fluid squirts onto the red hot rotor and bursts into flames!!
Cars with good rear weight bias (Cobras &Vipers) benefit from a little more rear caliper. I like 1” bore Dynalites or NDL’s and then use the Combo caliper as the P-brake only.
Jason