Haven't tried this, but looking at the documentation...
You get the node_mac in the initial request sent to your splash server.
When you craft your continue_url, can't you do it based on the node_mac?
You can optionally add a continue_url and a duration query to tailor the login process. To send the client onto their intended site, copy the user_continue_url value into the continue_url parameter.
I.e. instead of using the default user_continue_url value, choose the continue_url you want based on node_mac, for instance you could use the same host/path and just add the node_mac value on the end of the URL as a parameter.