Hi,
Yes, you can add labels to a category's flyout. It works exactly like adding them to a standalone flyout, you'd just add the label entry under the category's contents in the toolbox definition:
{
"kind": "categoryToolbox",
"contents": [
{
"kind": "category",
"name": "Some Category",
"contents": [
{
"kind": "block",
"type": "controls_if"
},
{
"kind": "label",
"type": "Whatever you want"
},
]
},
]
}