How do we politely respond to a journalist asking for questions we don't want published for now?

You can respond with something along the lines of:
“Thank you so much for reaching out. Right now, we’re not in a position to publicly comment on this, however we are happy to share some insight confidentially and off the record and/or discuss in greater detail in the future (and keep you in the loop with developments).” The confidential/off the record part is only useful if indeed you are comfortable doing so. If not, best to omit and just say you will reach out in the future.