Given a shader, it’s not too hard to find all materials that use (aka “own”) an instance of that shader. Here’s a Python snippet that does just that.

Note that I don’t check whether or not the shader is actually used. This snippet finds all instances, whether they are used or not (last week I posted another snippet for checking whether a shader instances was ultimately connected to the material).

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

def get_materials_that_use_shader( s ):	
	mats = disp( "XSI.Collection" )	
	oShaderDef = si.GetShaderDef( s.ProgID )
	for i in oShaderDef.ShaderInstances:
		try:
			mats.Add( i.Owners(0) )
		except Exception:
			pass

	mats.Unique = True
	return mats

#
# Find all materials that use a specific shader
#
s = si.Selection(0)
if s.IsClassOf( C.siShaderID ):
	mats = get_materials_that_use_shader( s )
	for m in mats:
		print( "%s in %s" % (m.Name, m.Owners(0)) )
else:
	si.LogMessage( "Cannot find shader instances. Please select a shader." )

# Material in Sources.Materials.DefaultLib
# Material1 in Sources.Materials.DefaultLib
# Material2 in Sources.Materials.DefaultLib
# Material3 in Sources.Materials.DefaultLib
# Material7 in Sources.Materials.DefaultLib
# Material6 in Sources.Materials.DefaultLib
# Material5 in Sources.Materials.DefaultLib
# Material4 in Sources.Materials.DefaultLib