Custom blocks and items count with a parameter called: 

"is_hidden_in_commands"

This parameter should allow for usage within commands like;

/setblock 
/fill
/give
/replaceitem

Here is the documentation:

How it works on items
How it works on blocks

After this being said, that means that when this parameter is set to false, it should be usable in all of these commands. However, it seems like this parameter is linked to the category parameter for items, which makes it impossible for creators to create items like command block minecarts, which cannot be obtained in the creative inventory, but can be accessed through commands.

The following code should mimic the behavior mentioned above for both, items and blocks:

"menu_category": {
      "category": "none",
      "is_hidden_in_commands": false
}

However, for some reason this only works on blocks.
I have attached a pack to help showing this, here are the steps to follow with it:
1. Download the attached pack
2. Create a new world and upload the given pack
3. Open the chat and type /give @s kai:test

Expected results:
Both, kai:test_item and kai:test_block should show up

Obtained results:
Only kai:test_block shows up
Kaioga's Bug Report Pack.zip