Doesn’t help much and I’ve not tried any transactions (they may have silently sorted permissions) but this works for checking balances if you’ve got a joint account pot id:
/pots/$pot_id
At least I can now warn myself that I’ve not manually put enough in my amex pot, better than nothing!
Actually just tested a deposit and it works! It seems the only thing missing is visibility of the pots. To get the ids I’ve just got them from the transactions for the joint account.