It is impossible to modify the loot tables of vanilla blocks.

Attached is a simple behaviour pack add-on 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).

However, using the datapack gives the errors:

[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

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.