I have redefined the default link "arrowhead" in my current project. Here is the graph code:
var paper = new joint.dia.Paper({
el: $('#paper'),
perpendicularLinks: true,
gridSize: 1,
model: graph,
//snapLinks: { radius: 5 },
defaultLink: new joint.shapes.devs.Link({
attrs: {
'.marker-target': {
d: arrowheadShape
}
}
})
});
Then this is the variable that holds the actual shape definition:
var arrowheadShape = 'M 10 0 L 0 5 L 10 10 z';
In this case I merely sized up the default triangle. You could replace this with any arbitrary SVG path you wanted.
ON EDIT: I believe I've misread your question. My approach redefines the default shape of the link's head, not the other icon that appears when you're dragging the end of a link around. My bad, sorry.