If you are asking for a favour from someone it would seem unsuitable. RSVP is what we use when we have invited someone to a party or something where we are doing them a favour and want them to let us know for our convenience.
For a guest speaker I think you should write a complete sentence encouraging them to let you know if they will be able to help you.
Agreed. RSVP is normally only used on something like a printed invitation to an event to which many people have been invited, where it’s important for the organisers to know in advance how many people they can expect to actually turn up.