There are many reasons I can think of why bolding / formatting specific entries in a list box could be useful! I have an app where I allow for quick browsing of my API features in a way very similar to the JMP Scripting Index. In order to allow for the formatting that helps with visual parsing I've had to re-create list boxes using Table Box and COL Box, wrapped inside of a mouse-box to be able to know which item is being clicked on and perform certain actions based on that (like disallowing certain entries from being selected).
It was a lot of work, but now it is simply a widget that I can embed in any script with one line of code, just like the built-in list box.
The two list boxes in this script are indeed TableBoxes wrapped within a mouse box, but the added ability for individual entry formatting is undeniable.
@SophieCuvillier , the short answer is no -- JMP does not provide any capabilities for us lowly scripters to modify the individual entries in a list box. They do have that capability, as the JMP Scripting Index does it with the list boxes they use.
If you're endeavorous you can make TableBoxes behave like a list box, but it does require a bit of work. I cannot give out my code for it per my companies regulations, but I'd be willing to help answer any questions if needed.
Jordan