Hey John,
We looked into doing this, even have a prototype on the shelf, but encountered an issue.
It's pretty standard design to put a turn on time delay in buck converter circuits, to give the system enough time for input power to become stable. It's a pretty short delay half second or so. But; the result is, running lights work great but turn signals may not even flash at all, or it they do it's a way too quick little blink.
The thing we never evaluated, because we never found an off-the-shelf buck converter with a short enough turn on delay, is; does the repetitive turn on surge kill the buck converter in a short time.
Add a couple of turn signal relays in your design, old school but time-tested, rugged and waterproof (if you get the correct ones).
Regards,
Mike W