It is impossible to modify the loot tables of vanilla blocks.
Attached is a simple behaviour pack that attempts to change the loot table of a sea lantern to always be a sea lantern (instead of prismarine crystals, as it is in vanilla).

Steps to reproduce:

1. Load the attached behavior pack.
2. Add it to a world
3. Enter a world with the Content Log GUI enabled.

Observed results:
Multiple content log errors appear:
[ERROR] [Blocks] block_definitions | ... | blocks/sea_lantern.json | Identifier not allowed to use Minecraft namespace
[ERROR] [Blocks] block_definitions | ... | blocks/sea_lantern.json | Failed to register Block with identifier: minecraft:sea_lantern
which signifies that vanilla loot tables cannot be overridden.

Expected results:
No content log errors should be thrown, in addition to the modification of vanilla loot tables being supported.

Notes:
Vanilla block loot tables were previously able to be modified as long as the namespace of the identifier tag was changed to anything other than "minecraft" (e.g."minecraft:sea_lantern" => "a:sea_lantern"); see MCPE-53306. However, this is no longer possible either.