I've had our reservations linked with friends/family on other lines. All I had to do was give their booking number to my TA, and they gave my booking number to their TA. This was not on Royal Caribbean, but we've done it on several other lines, including "sister" line Celebrity. As you want to eat together, I would not wait until you board. Try again and let us know how it turns out.
Edited to add:
Neil, we posted at the same time.
Out of curiosity, why would this not be possible through an online booking? Wouldn't they have a booking number? And, if they did, would the other person's TA still be able to link them? Again, I'm just curious as I know little about what goes on behind the scenes with bookings.