Joni Mitchell does not provide personal contact information, but would you like the information about her agents? They could possibly forward a letter to her.
Her agency (which is actually located in Canada) is:
S.L. Feldman & Associates (SLFA)
Vancouver Office:(NNN) NNN-NNNNWest 2nd Avenue, Vancouver, BC, V6H 3Y4, CanadaPhone:(NNN) NNN-NNNNFax:(NNN) NNN-NNNN/p>
Toronto Office:XXXXX Toronto, ON, M5G 1G7, CanadaPhone:(NNN) NNN-NNNNFax:(NNN) NNN-NNNN/p>
I hope this helps. Good luck contacting her!