The add on thing for PS3 BC is pretty much unlikely. It would need to feature all the PS3 main hardware (Cell+RSX+XDR+GDDR3) also because USB has not enough bandwidth as a bus (peak bandwidth for USB3.0 is 625MB/s), it would need its own power supply.
Basically it woud be like buying a PS3 without a Blu Ray drive and connect it to the PS4.
At that point it makes much more sense to simply make a PS4 model with hardware based BC at a higher price (people who want the extra feature without compromises pay for it, seems pretty logic and fair to me). But we are ruling out other specific solutions like Gaikai and hybrid emulation.
Honestly I don't think that Sony wants to lose compatibility with all the contents available on the PSN.
wouldnt they just be able to sort of retrofit/emulate certain parts of PS3 (RAM/RSX) to the PS4 hardware and then just use the exclusive PS3 hardware such as cell on the add-on? Also... they don't have to use USB as the connector.. could use some other proprietary connector for it...