Our RO plant doesn’t have a neutralization tank for our spent CIP solution. Can we just neutralize while circulating through the membrane system?

That is not recommended.  A high pH cleaning solution is designed to dissolve membrane fouling/scale that is soluble in basic conditions.  A low pH cleaning solution is designed to dissolve membrane fouling/scale that is soluble in acidic conditions.  Neutralizing these cleaning solutions will precipitate most of the materials that were dissolved back onto the RO membrane surface, rendering the cleaning ineffective.

If you do not have a neutralization tank, you should neutralize the RO cleaning solution in your CIP tank, and drain.  Then fill the tank with permeate water, circulate through your system to displace more RO cleaning solution from the membranes, neutralize again, and drain again.  Repeat until you measure a displaced cleaning solution pH between 5 – 9, at which point, you can flush the membranes with feedwater to drain.