When you say "custom splash page" do you mean creating it from the Meraki splash page settings? Or do you mean you want to create your own from scratch?
You can however create own splash from what Meraki provides. Then in access control make the splash page sign on with Meraki authentication. From there you can choose what self registration settings occur.
I might need a little more info on what you mean by self registration, but I can help with the custom splash page part. Once you select one of the splash page options on Wireless > Access Control, you can set up your splash page. Navigate to Wireless > Splash Page and there is a section called Custom themes. Click the link that says Create something new, choose a base, and then you will have a copy of the theme. If you hover over your new them it will highlight, then you click on the pencil to the right to edit it. You can pretty much do what ever is possible. You can upload your own html, css, and image files and have at it. You can also edit any of the html pages and the main.css file. And if you mess it up bad, just delete the copy and start over.
I would select Sponsored guest login under Access control and your choice of Modern or Fluid theme to get a page up, and then tweak it in an outside editor until you get your desired look. Hope this helps! I will attach a screen shot of our current splash page we are still tweaking.
