Is there a way to make a block placeable in adventure mode

Yes, even in vanilla, you can use the “execute command /<string>” block with the /give <player selector> <item>[minecraft:can_place_on:<param>] <number of item> command. This command will give the player a block that can be placed on other blocks, or all of them. You can also make it appear, instead of giving it directly to the player, it's cleaner. You can find more information here : https://minecraft.wiki/w/Data_component_format?section=15#can_place_on

link in bugged, real link