Why not to use a P-Channel MOSFET in this circuit?
I suppose it could be made to work with a P-channel MOSFET, but I'm not sure what real advantage that would give. Is the voltage drop of about 0.2 volts over the transistor a problem? Probably not, and a P-channel MOSFET is likely more expensive than a PNP BJT, and this circuit is simple and works great.

To highlight one design hurtle to using a MOSFET, what happens \$V_{self}\$ is less than \$V_{bus}\$, but not less enough to reach \$V_{th}\$ of your MOSFET and turn it on? Also, the gate would be driven though a resistor here. If \$V_{self}\$ fails, will you be able to drive the gate low enough and fast enough to maintain power?

Not unsolvable problems by any means, but why bother thinking of them when the BJT works fine?