Small panel for Nuke to add a user knob to a selected node which is linked to a certain knob of another node. This basically works like the Pick Button, but for all nodes of the comp instead of just the containing nodes.

link_knob_panel.py

# TODO:
# - convert label to proper knob name, e.g. w/o spaces
# - check if link expression is valid
# - implement a node and knob picker

if len(nuke.selectedNodes()) == 0:
    nuke.message('no node selected')
else:
    n = nuke.selectedNode()
    
    p = nuke.Panel('Link Knob')
    p.addSingleLineInput('label', '')
    p.addSingleLineInput('link expression', '')
    ret = p.show()
    
    if ret:
        label = p.value('label')
        link_expression = p.value('link expression')
        k = nuke.Link_Knob(label)
        k.setLink(link_expression)
        n.addKnob(k)