Thanks for your detail description.
I am not sure whether we will support for more parameter in future, but I will let our developer know this requirement.
For now, I viewed the code, it looks we will parse a parameter named: "CustomParameter" from HttpContext, and then assign that into the GetMapRequest.CustomParameter. I think you can use this parameter for your requirement.
You can get the GetMapRequest.CustomParameter in function GetMapCore, then parse it and implement your custom logic here.
Wish that's helpful.