I going to book an OW award ticket with AA miles, I know that I can put the trip on hold for 14 days.

If the first part of the trip is say available now, can I hold those flights and put in dummy flights for the rest of the booking then change the dates and flights of the dummy part at a future date (within the 14 days)?

I think I have found the answer to my own question by doing another search and using the word dummy. I feel like one at the moment.

I would say the answer is Yes.