I've put together a hacked addition to the right-click menu (using a previous @dardoro project) that seems to able to move custom blocks to the top of the list
I've no idea if it's safe to use on a complex real project so use at own risk
https://snap.berkeley.edu/snap/snap.html#present:Username=cymplecy&ProjectName=CustomCommandMenu.moveToTop
[edit2 - now works with command, reporter and predicate custom blocks]