I feel like that would be unnecessary. Many champions have similar ability reactions as binding light. For instance; Shyvana's dragon breath, Mordekaiser's siphon of destruction, Vel'koz' plasma fission, and many other abilities make noise or the ability itself stops moving when it collides with a target. If ALL of these abilities had a similar "passive" as the one you are proposing, many strong champions, could gain an even larger advantage over the lower-teir/weaker chamions.
For the idea of her getting a shield from prismatic barrier, in short she already has a shield, but even just one. As I'm sure you probably know, when the initial amount of her shield gets depleted by enemy attacks it refreshes upon return, that in it of itself is extremely strong. Finally, Lux does not need anymore durability, that's not what she is suppose to do. Lux is a strong mid-long range APC(or support) who has enough utility if used properly to keep her distance from most attackers.
Not only that, but riot is trying to give assassins an edge or some crap in these latest patches, there's no way they'll undo their most recent work. :/