Have you restarted the Concentrator service? If you have, can you show the Meta view from and event from a recent time - since you changed index-concentrator-custom.xml and restarted the service - that shows that meta key being filled?
In looking at the table-map file the proper variable is sig.id not sigid. In your index-concentrator-custom file the name should be sig.id. Make the change, restart the concentrator and your values should be indexed.
Yes, so Dave's suggestion above is correct. The "sig.id" is the proper meta key for SignatureID. If you take his suggestion above and change your key as he described, it should work fine (after a service restart).