I'm looking to make a sublist field read only, or inline, on a client script, is that possible?

yes, look at

updateDisplayType

from serverWidget? that is only available in SuiteLets and Before Load UE

sorry, you're right that isn't available from the client side

client script - pageInit Column.isDisabled will work.

the hard part is that the requirements are to only disable the field based on a check box on the item record, so the column field is disabled on a line by line basis.

use the lineInit entry point instead so you can disable or undisable the column as necessary

I was thinking that was my only option, it's a bummer that disabling the column affects all other lines but I guess that is the life I chose