PNP Transistor - Use IC 555's internal resistance as pulldown?
No, it's a terrible idea. There's more than just the three 5k resistors connected between Vcc and Gnd inside the 555. In fact, the PNP B-E junction will pass enough current to operate the 555, so it will never actually turn off.

You could do it with a P-channel MOSFET, however.