The ICU4C error callback framework should be ported to ICU4J. There is already an incomplete private implementation. It just needs to be cleaned up to verify that the framework is working as expected, and it should be made public.
Some of the ICU conversion data driven tests already expect the error callbacks to exist. When this API becomes public, the tests will have to be modified to run these error callback cases.
This is a desired feature from an Apache developer (IBMer) that wants XML escapes.