Relax chief.
You have to realize that our cars are at the youngest 18 years old, and z3's are already reaching 13 years of age. There should be an obvious correlation. People have had plenty of time to think of bastard swaps of suspension and drivetrain parts, but the truth is, there just aren't very many wrecked z3's to pick from. e36 sedan's are much more prevalent and thus, there are many more "realistic" options.
To put it simply, the z3 is a bastard child in and of itself. It uses e36 front suspension pieces with essentially e30 m3 rear suspension. e36 front strut assemblies and control arms can be fitted onto e30's, so it seems that z3 bits wouldn't be much different. Swapping rear suspension pieces just seems pointless from a cost standpoint. I understand using e30 m3 rear trailing arms and hubs for 5-lug conversions, but without that incentive if really isn't worth the cost.
Anything is as easy as anything else. If you consider easy buying a complete e30 m3 rear subframe and having a shop install it for $2k, then that's one thing. I wouldn't call any major change in suspension pieces easy unless you are paying someone else to do it.
Do you have a wrecked z3 m-coupe in your back yard? I can imagine it would be near impossible to find one wrecked, and even if you did, you are going to pay an arm and a leg because the parts are "really sweet" 'M' parts.
I think that the alloy m52's found in the z3 2.8's are "really sweet" but it isn't worth the cost.
This board moves slow, we don't have a large group of regulars members, and many times only relevant threads really gain interest. I don't see this being one of those threads unless you have gold plated rear trailing arms for sale for $50 shipped.