Well, that was easy. The command in the Dactyl Nest summit is at offset 0282.
Address: 7F01DF
Bits:
80 Crono
40 Marle
20 Lucca
10 Robo
08 Frog
04 Ayla
02 Magus
Set the bit to lock a character. 84 would lock Crono and Ayla. More examples:
16 = lock Robo, Ayla and Magus
E0 = lock Crono, Marle, Lucca
98 = lock Crono, Robo, Frog
The character will be locked into the current slot they are located at in the active party. If Ayla is in slot 2 when her bit gets set, she will be locked in slot 2. If you wanted her locked into slot 1 or 3, you'll need to shuffle your party members with AddPC/Move PC to Reserve commands. Take a look at the code right above offset 0282 in the Dactyl Nest summit event to get an idea of how to do that.