This is the safe and appropriate route. When there are issues like this, it has to go up and down the official food chain. If you try to take a shortcut across the org chart and deal with it directly, you'll alienate all parties involved. (Been there.)

Of course if you're in a small company, things are much less formal. That can make resolution more difficult. But you can still say "let me check my delivery schedule -- okay, I'm full-time on xxx project until next week, and then I can schedule you in." Message delivered, without being confrontational.

I would avoid saying "I don't know" -- it sends the wrong message about the currency of your skills and/or your level of motivation.

It's not necessarily a bad thing for top brass to see you as the go-to guy for their new tech. But if it interferes with your assigned duty schedule, i.e. your real job, that message needs to be delivered.

My 2 cents'.