Remove Umaro and Hidon???

The address Xenovant provided for Umaro is after the screen pops up and you choose to remove what's inside the carved bone. Erasing this event, or setting the event bit $27A in a previous event also takes care of the recruiting scene with Umaro while at the same time avoiding the text box to be triggered.

Code:

CC/D6EB: C0    If ($1E80($27A) [$1ECF, bit 2] is set), branch to $CA5EB3 (simply returns)
CC/D6F1: B2    Call subroutine $CACA8D
CC/D6F5: 4B    Display dialogue message $05F4, wait for button press              
What's with this carving?              
Looks like bone…              
Something…in that eye…              
… Magicite?!
CC/D6F8: 92    Pause for 30 units
CC/D6F9: F4    Play sound effect 103
CC/D6FB: 55    Flash screen with color component(s) 8 (Blue), at intensity 0
CC/D6FD: 94    Pause for 60 units
CC/D6FE: 4B    Display dialogue message $05F5, wait for button press              
Remove the Magicite from the eye of the carving?              
^ Yes              
^ No
CC/D701: B6    Indexed branch based on prior dialogue selection [$CCD709, $CA5EB3]
CC/D708: FE    Return

Hidon's cave is made accessible in the world map via the series of events in Thamasa, once you arrive to it in the WOR with Strago and Relm. I believe that the address that xenovant provided was correct here. You can either remove the check for strago and relm in the party during the world of ruin the first time you enter thamasa, or erase the whole event altogether.