Could the W65C02S drive a LED on its databus?
I'd be surprised if you fried the data bus by overloading it. It's not a very powerful output drive according to the numbers you posted. If you applied 12V, then you probably fried it.
The datasheet spec is only a minimum so you can design your circuit to that value and the chip is going to at least that value or better. The maximum current might be determined by other specs, but it is unlikely the maximum is going to be significantly larger than the minimum as the chip was designed using mosfets of a given geometry, so it really comes down to the process variation.
As for driving leds, it's not impossible, but probably not a great idea. The chip looks like it can sink current better than it can source it, but you'd want the led to be on when there is a logic '1' on the bus. If you load the output with a led, you'll decrease the output voltage of the processor which might affect the rest of the circuitry. As well, the data bus is bi-directional, so other devices on the bus would need to be able to drive the leds.
I'd suggest you use a buffer like a 74HC244,74HC541,74HC245 to drive the leds.