Is it safe to externally power a net normally powered by a SMPS (3.3v buck)?
It's a synchronous buck, so it may get confused and try to power the circuitry that's powering it.  If it does that it'll become an unregulated boost converter.

I would put a voltmeter on the PVIN lead, an ammeter on the lead to the 3.3V net you want to power, and carefully bring the voltage up.  Or keep the voltmeter and try to bring it up through a series 47\$\Omega\$ resistor (or whatever value you think is appropriate).  That way if anything tries to suck too much current it won't act like a short.