She's never hidden/behaved fearfully, never bitten aggressively--She did chow on my foot under a blanket once, but never again after I told her no--she's fine when people come over, lets them hold her, pet her, etc. Just in general Feather is an incredibly calm, incredibly sweet cat.
I don't know anything about therapy cats or whether it's possible with most cats. I have thought about the topic, because I am currently harness training my cat, so if she adapts to new environments well enough it could be a possibility for her. So I'm definitely interested in responses from people who might know about this topic.
With that said, please keep in mind that a cat being well-behaved, social, and not fearful at home does not necessarily mean it will behave the same way in a new environment. Most domestic cats, even those that are adaptable, don't feel completely at ease outside their home turf.
Take my cat, for example. She adapted to my house very quickly, and since then has not been easily scared. She has never been aggressive. She has never hissed, and has only play scratched/bitten us a couple of times in the very beginning but very quickly learned not to do it. She doesn't like to be picked up, but aside from that, she is very friendly. She is perfectly comfortable around any new person that comes over, she sticks around wherever there is people, allows them to pet her, and will even lick their noses if they come close enough to her. Her body language is totally relaxed when there are people around, she lays down with her belly exposed, closes her eyes, etc.
We are harness training her, and she is totally different outdoors. She gets alert and fearful and is very cautious. She will not relax and lay down. If she sees people, she freezes up, and if they make any loud sounds or walk towards her, she gets scared and starts running back towards the house. This makes the training sessions hard, because there's almost always someone out there and walking a cat on a leash draws attention, people want to come over and make conversation or pet her and she won't have any of it.
Now, we haven't even been doing this for a month yet, and we don't do it everyday because it depends on the whether, what's going on outside, etc. I think this is something that can be changed, and eventually she will be comfortable around new people outside the house, too. However, even then, if I took her, put her in the car, and drove her to yet another entirely new place, she would be uncomfortable about the new people there, yet again. So while perhaps this is possible with cats, I think it's extremely hard, and you won't be able to make that call just based on your cat's behavior inside its own home.