I don't think the US Regs make any distinction between a beacon or strobes. The normal 3 nav lights are reqd i.e Red, Green and white but only an anti collision lighting system is required beyond that. That just has to meet field of view, flashing and intensity requirements and they can either be red or white.
The CASA regs do state that "unless CASA otherwise directs". Maybe if the aircraft is certified to Part 23 in the States, CASA just bypasses that reg?
To install a beacon "aftermarket" would involve getting an Engineering Order then I would think also a re-weigh of the aircraft. Beacon units can be quite heavy and could change the aircraft empty c.g position quite significantly depending on where it is located.