Does anybody know what べた打ち means, and what the kanji for べた could be? The context I heard it in was the upright playing style of taiko.
Depends on if its a technical term or just a colloquial expression. I'm no expert on taiko, but this PDF written by a Japanese taiko maker says that ベタ打ち is the opposite of the ideal way of using a stick to hit a taiko.
According to them, you're supposed to hit the surface of a Japanese taiko with a snappy motion so that the tip of your stick touches only for a very short period of time. This is supposed to make deeper resonance.
ベタ打ち is the poor form of hitting that does exactly the opposite (or so the PDF says). You're not snappy enough so not only the tip of the stick gets landed but also the side also touches the surface, killing resonance and potentially damaging the surface of your taiko.
But I must admit that I didn't know ベタ打ち as a technical term before I googled it today. And actually I was about to post an answer explaining the more usual meaning of this word. Thank God I googled it just in case!
Anyway, if the speaker is not a taiko master, another potential case is that it's just one of the usual meanings of べた, which doesn't have a kanjified version as far as I know, plus 打ち. The ones I'm aware of that are often followed by 打ち are
The third one is sort of technical jargon and means that you copy&paste data from one place to another or move data without changing its content to another file etc. So this can't have anything to do with Japanese taikos. The first one might be it because it kind of sounds like a sequence of rapid strokes. The second one seems to be why the bad form of stick work is called ベタ打ち... So if it's not the technical term, the speaker might have meant fast beat drum work.
I don't think there's kanji for べた as it is 擬態語.
I actually didn't know that it is used in the context of playing taiko, but according to this, it means hitting the taiko surface with the long edge of the stick to maximize the contact surface.
The way I understand べた in べた打ち is that it represents stickiness. So for example in type-setting it refers to putting letters next to each other without line space (thus letters are sticking to each other), and in text typing it refers to entering text without editing (and thus adding space), and in construction it apparently refers to driving lots of stakes into the ground right next to each other.