In this month's release of the two plugins, there are a couple of features that can help:
adding an "extra message" that should work in the legacy and pipeline invocations, which is a string you can provide and will be attached to the standard build starting/completed notifications. Your job could generate it somehow (easier in a scripted pipeline). Not sure if it supports groovy variable expansion like many other plugins do, but I'd assume that is relatively easy to research and implement if needed. This might be flexible enough for passing the extra data.
more for the pipeline context, calling `ircNotify customMessage:"Some text"` should just post that message verbatim (which again you can create in the pipeline) at that point in execution