Quite frankly, yes and no. Let's say the person playing with Billy is named Jay. If Jay were to open the chest that Billy put the custom mob head in, the chest would EITHER appear empty OR would have a missing texture icon in the slot where Billy put the head. If it appeared empty, if Jay were to CLICK ON the slot that Billy put the head in, he could still pick it up/move it into his hotbar, but on Jay's screen it looks like an empty slot named hgkhjkg.txt or something.
If the modded item was a SPAWN EGG, it would also appear as a missing texture or empty slot that could be picked up/moved into hotbar. If Jay were to USE the spawn egg, nothing would happen on his screen, but on Billy's screen it would work- it would appear as Jay was spawning his own mobs.
hjgkkjhl I hope I wasn't too technical for u