Using signature options and conditional expressions, what UI properties can be controlled conditionally? Especially interested in making a readonly field editable.

You can hide/display a textbox or section by creating a new sig option and then applying the sig option on to a textbox with an associated security group linked to a conditional expression thereby controlling the hide/display functionality. to make a readonly field editable conditionally, you can use the Global security group option under security group or in data restriction tab of a security group. Unless that field is controlled by a class file which needs to you override the control by extending the field class or creating a new automation script.