If you hung an orange flag over the thing, I really don’t see where the cyclist would be in any culpable, particularly if it only extended three feet. Tack on a rear-facing camera, and I don’t see where a close-enough-to-get-scratched motorist would have any legal standing at all.
A reasonable judge might still hold you culpable or not.
However, it may convince someone who is royally peeved that your device scratched the crap out of their car and use that as an excuse to get out of their car and attack you. I don’t see this ending well for you if the friendship blades got friendly with another vehicle