If a user wants an explicit selection of a single object type and sets the filter for it, he should not be forced to setup a custom geometry filter to filter for the same object type. Decal widgets seem to be connected to the filter with number 268435456 which stands for Phantom objects, at least i can use rs.GetObject with this number and it allows only to select the decal widget.
Decal widgets deserve a more meaningful depiction and handling in RhinoScriptSyntax. If a user interates through all objects and a decal widget is found, query the Object type like this:
import rhinoscriptsyntax as rs
obj_id = rs.GetObject("Select widget", 268435456, True, False)
This prints 0 which stands for “Unknown Object” according to the help.