ASDF checks to make sure all of the initargs are defined when parsing a defsystem. This is good for catching errors, but is terrible for extensibility. This means that any attempt to add additional metadata will be backwards incompatible.

I can think of two ways to fix this:

1. Add a "garbage can" slot to component that will be filled with a property list, and allow programmers to do whatever they want here. This doesn't seem great to me.

2. Add a new defsystem parsing error class that is unknown-component-property, raise it when an unknown property is encountered and allow the user to continue, discarding the initarg and accompanying value.

The second alternative is what I favor. It isn't great, because it will only maintain extensibility going forward, but I think it's the best we can do.

I suppose that we could also add an initarg to tell ASDF to continue such errors silently. I'd be inclined to suggest that this take an ASDF version expression as value, so that the error is quietly ignored only by ASDF versions below that. This means that the property will start to be checked when it has become authoritative.

Note that for one's own system and component subclasses, the set of initargs can be extended without any monkeying around.