Given something like this:

Here’s how you get all shaders under a light, even the disconnected ones:

from sipyutils import si		# win32com.client.Dispatch('XSI.Application')
from sipyutils import log		# LogMessage
from sipyutils import C			# win32com.client.constants
from sipyutils import disp		# win32com.client.Dispatch

si = si()

def dispFix( badDispatch ):
    import win32com.client.dynamic
    # Re-Wraps a bad dispatch into a working one:
    return win32com.client.dynamic.Dispatch(badDispatch)

oLight = si.Selection(0)


#import win32com.client
oDisconnected = disp( "XSI.Collection" )

if oLight.IsClassOf( C.siLightID ):
	for oShader in oLight.GetAllShaders():
		oOut = dispFix( oShader.Parameters( "out" ) )
		if oOut.Targets.Count == 0:
			oDisconnected.Add( oShader )
			
log( oDisconnected.GetAsText() )

Hat tip: Matt Lind, who provided the GetAllShaders answer to the question “how to get all shaders in a light, even the disconnected ones”