So lets say in a game, a player claims that I am scummy because of something I did. Then I point out that in a previous game, I was accused of the exact same thing and in that game I was town. Is that a valid defense because it shows that the particular action is something that I do even when I am town?
as a defence (i.e. "this would be a scumtell from most people, but isn't from person X").
However, I don't believe self-meta is valid for anything. Meta normally only works on people who aren't aware with it, so someone making a meta argument on themselves is pretty much intrinsicially invalidating their own argument. (As Wisdom's mention of trust tells is suggesting: if you're aware that there's something you do that indicates you're a particular alignment, and yet you
don't
do the "townish version" of it as scum, you're effectively not playing to win. So the only valid self-meta would be something that you're physically incapable of faking as scum despite being easily able to do it as town, and I find it very hard to see that situation arising.)
The specific example in the OP seems more like a meta-meta tell to me, though (which is something I've been meaning to write about: if someone's meta describes the way they in particular act, their meta-meta describes the way that other people act towards them in particular). Knowing your own meta-meta is very useful and one of the tools that's been more useful for me in catching scum. I haven't seen it used as a defence before, but there's no obvious reason why "people who say X about me are usually wrong" is necessarily an invalid statement, nor necessarily anything that the person being spoken about has any influence on.
Self meta can be used in the case of a defense but not when you are trying to say you are towny. If you are trying to say that you are towny because of it then there is incentive to fake it. But if all you are doing is defending yourself then all you are doing is saying that it is not scummy which is perfectly fine.
The example I gave can be summed up like this:
Redacted #1: You are scummy because you did X
Redacted #5: You guys went after me for that exact thing in another game and it didn't work then.
The purpose here isn't to actually read someone based on meta. The example I gave was not using meta to claim that someone was either town or scum.
However, meta can be used to show why a particular argument is invalid. For example, someone says that you are scum because you were wishy washy. You then point to multiple games as town where you were accused of being wishy washy. Clearly their argument is ridiculous.