That kinda goes outside the scope of things, as Placeables is meant to deal with an ever-changing inventory. For example, when you handcraft a single water pump and don't want to go back into your inventory menu to grab it after its done crafting, or when you use a mod like Bluegistics to completely change what items are in your inventory all at once.
Though I think the biggest problem would be polling the players inventory for locked inventory slots and keeping track of all that. The only reason Placeables doesn't have much of a performance impact is because the API gives a way to get a list of all items without having to manually check every stack and add everything up yourself. It will just magically tell me that the player has '1332 iron plates' and stuff like that. Without that functionality from the API, the mod wouldnt be viable.
Looping through a players inventory to find out what slots are locked would for sure be a big performance hit. Plus, if all it did was show items locked in your inventory, it would just be more of a big moveable hotbar, which could still be a good idea for a mod though.
Blacklisting items, as in the original idea you had, I do think would be quite good for this mod to have though. I don't think it would have much of a performance impact, I just need to eventually have the time and willpower to try to shove it in XD
