Good news for owners of an ACA1230: I just got the 42MHz version to work stable on the ACA500. I also have a 56MHz version here, but that has an overclocked 40MHz CPU that does not run stable at 56MHz. It works with ice spray, but I cannot deduct a general compatibility from that.
I didn't take the time (yet) to test the 28MHz version, but I'm confident that I can get this to work as well. Syncronous designs are now "not generally incompatible" any more. The logic can now handle a total of three different timing models that should cover all possible cases.