To send a custom block to the bottom of the category, move it to a different category and then back again.
Or duplicate it and delete the original
Just tried doing this with my comment block but it ended up back in the same spot after I'd changed it to motion category and then went to that category and changed it back
It even maintains its relative postion of 4th from bottom when in new category
That would remove all references to the block from the project so not a good idea
Oh, that's a pity.
You might be interested in this
I've put all the code in a block named "enable move block to top", exported it, and imported it into my project. It activates the right-click button menu option.