You totally can use RFC6979.

BIP340 even says how to do it: https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki#alternative-signing

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It's not the standard though so you can't rely on it

I just added RFC6979 logic to my BIP340 implementation:

https://github.com/conduition/musig2/pulls/2

Seems reliable enough to me!