it sounds feesable, just how big is the rust spot? you said you could push a finger through it(sounds bad). i've used JB Weld on my gas tank and it held for about 3 days. of course a gas tank has more pressure than an oil pan. if it were me, i'd give it a try. if it doesn't work, then you know your other option. the only thing i would worry about is the rust on the inside of the pan. it could do some serious damage to the internal parts.