I think the choice has nothing to do with “une fois”. “Une fois” does not put the context in the past, either.

I Taken strictly, your sentence means that when a given time comes up, in the future, you will be at the bank, and then you’ll have the opportunity of telling them what you kept on having done until you got at the bank and intend to keep on having done. This is so because the present continuous of “are having done” (to have sth done) situates the action in an interval of time that spans a little time in the past, the present moment and some indefinite period of time after that. As the action is not hypothetical but declared as being taking its course now, you can’t use a conditional in French and you have to use instead a form that renders that idea of real action. You can use the simple present for that, as you do in the first option (as in “It is raining” ==> “Il pleut.”).

• Une fois que tu seras à la banque, tu pourras leur dire ce que tu fais faire.
  (For example, “Je fais faire des réparations à ma maison.”)

There is an alternative form that translates well the present continuous.

• Une fois que tu seras à la banque, tu pourras leur dire ce que tu es en train de faire faire.
  (For example, “Je suis en train de faire faire des réparations à ma maison.”)

II However, it is possible that you do not give that sense to this sentence; you might want to say that once you are at the bank, you will be able to tell them what from that moment on you will be having done. In other words you put into “are having done” the intention of an action to come. You are then using a present in the place of a future continuous to say effectively this:

• “Once you are at the bank, you will be able to tell them what you will be having done.”.

In this case you must translate as follows.

• Une fois que tu seras à la banque, tu pourras leur dire ce que tu feras faire.

or, equivalently

• Une fois que tu seras à la banque, tu pourras leur dire ce que tu vas faire faire.

You cannot read that into the present in French, so you must use the future or a form that carries the idea of an action taking place in the future (“aller + infinitive of verb”).

Here is a couple of suggestions:

Une fois à la banque, tu pourras leur dire/expliquer ce que tu veux faire faire.

Une fois à la banque, tu pourras leur dire/expliquer ce que tu voudrais qu’il fassent.