When entering a future transaction in an investment account you are prompted as to whether you want to create a scheduled transaction or not. As scheduled transactions are not currently supported on investment accounts, you should not be prompted.
I just tested this in master branch, and you actually can create a schedule in an Investment Account, but not an investment transaction. I entered one event and it did create the transaction, decreasing the balance by the amount of the payment (as well as transferring the funds to the set caategory) but it's not clear where this decreasing balance is tracked. I suppose I should open a separate bug for this. I'm not sure if this bug should be to not prompt the user, or make it a wishlist to allow scheduled transactions in Investment Accounts.