I don't honestly know why they skimp on the inductors, in all of the car chargers I've seen they seem to be about half the 'magnetic capacity' (for want of the correct terminology) that they need to be, so something interesting begins to happen with the 34063.
You find that the overcurrent circuit starts kicking in and the whole thing starts to oscillate at a higher frequency than the internal oscillator of the 34063. So, technically its still working as a Buck converter but sadly in a mode it was never meant to.
You find that because of the high current and high frequency, the switching losses in the 34063 start to mount up to something much more than they should, and more than the device was intended to handle. The result, thermal shutdown and/or smoke/flames. The result of the latter is sometimes the car battery voltage briefly appearing on the 5v USB connector of the charger
Bad times.
I've fitted a 5.3v Zener (and a much better inductor) on the output of my in-car charger, this is intended to protect the phone should something like this happen. Also sometimes I've found they fit oversized fuses in them, 1.25A is a common one. bearing in mind the efficiency of these ICs (even the cheap ones), you see that Max output (5v 0.5A = 2.5w) doesn't equate to Max input (12v 1.25A = 15w). I've downsized the fuses in mine.
Anyway, back to YOUR circuit (I get carried away sometimes)..
Yes, I've looked at your design and the only thing which would concern me is that you might get some instability due to the Pi filter prior to the V-Reg transistors. The 34063 is expecting to see an instant change at the output when it increases the PWM, even though your inductor is small you will probably still get some lag, during which time the 34063 will kick the voltage much higher.
The overall result may be a slight 'ring' on changing loads/voltages (I've had this) or it could be a complete "PSU-throwing-a-wobbler" type of scenario. You wouldn't know until you built it in real life I think.
Something to consider maybe, perhaps you could split R5 into 2x 50k (47k) resistors, then add a small (size to be determined in spice/real life) capacitor from the centre point to the top of C3?
This (although inelegant, its simple) would provide AC compensation to your DC feedback loop, hopefully making it track a little quicker without causing instability.
Its been a while since I designed a switching PSU so I hope you can bear with me
Also, get those toroids wound, I don't know why, but there's something satisfying about doing so! (also they're less magnetically noisy which your MW sets will thank you for). Do you have an inductance meter? I built the DIY one using a PIC which was doing the rounds some time ago, it works brilliantly.
Dave.