Raycast equivilent for UV Editor

Coding Python Support
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8

You could give an eye to TexTools for Blender, especially utilities_uv.py file.
HTH

EDIT: All in all I think you’re looking for st. like:

    bpy.ops.uv.select_loop()
    bpy.ops.uv.select_linked()

first one will select the island loop under your cursor; the second will extend all the selection to the whole island, so you have the full loop.