Retrieval of Query Parameters in the ID Provider

The URL that goes to the SP that initiates a login to the ID Provider has Query Parameters on it that I would like to retrieve in the ID Provider when I do the SAMLIdentityProvider.ReceiveSSO(…) Can you tell me how I can retrieve that information?

Do you mean you wish to retrieve query string parameters at the endpoint where you call SAMLIdentityProvider.ReceiveSSO?
Assuming so, you can do this by accessing the Request.QueryString property on your page.