Need to pick up a new ballast resistor, it's about a twenty minute drive each way. Is it safe to temporarily attach a jumper wire across the resistor, or will it fry my coil and points ?

No worries. Just don't stop for coffee. Let it cool off for as long as you can stand before you come back. 

Other options include, open the hood and blow on the coil or buy an extra large Slurppie and pour half over the coil before you come home. Drink the other half of course....

Get a 3 ohm coil and you won't need a ballast resistor.

get a 3 ohm coil ? Damn, good idea but I ordered a new 1.4 ohm coil yesterday.

Didn't have to run without it. Found the original ballast resistor in box of parts. Tried it yesterday, but car still wouldn't start. The coil was reading about 2.5 ohms, a little high, so ordered a coil and resister just in case. Today checked the original 44 year old ballast resistor, couldn't get a reading until I worked it over with fine wire brush and Deoxit; after that it read 1.8 ohm, OK per FSM. So I removed the resistor that was in there and installed the ancient one. Also worked on the contacts on the PS20 coil, after that it read 1.6 ohm, so OK. Didn't need a new coil. Also used Deoxit on all connections including the socket on the coil for the power wire to the distributor cap, there was corrosion in it. Hooked it all back up and the car is running again. More power at low end, too.

About the ballast resistor that came with the PS20 coil: when I first installed it, about 6 years ago I noticed the contact prongs were loose in the ceramic body. I seemed odd, but I foolishly thought it was OK, since it was new. Today I looked at the underside of it, the resistor was broken completely. Possibly vibration, but I think it was previously cracked or weakened by just the motion of placing wire connectors on it. Suggest use a Standard Motor Products RU13 or similar instead, not expensive and way better.