It's hard to recommend an approach without knowing more about the setup and the kind of business it is.
Let's pretend it was a retailer with lots of stores. In that case, they should be using configuration templates.
https://documentation.meraki.com/zGeneral_Administration/Templates_and_Config_Sync/Managing_Multiple...
With a template, any change you make to a template will be applied to every store bound to that template. So you are guaranteed all your store configurations will be identical, apart from the subnet for the local LAN.
You could also create your own captive portal, and point all of the branches to that.
https://meraki.cisco.com/lib/pdf/meraki_whitepaper_captive_portal.pdf
You could use an external splash provider. Splash Access is super popular.
https://www.splashaccess.com/
You could use the API and programmatically configure all the networks the same.
https://developer.cisco.com/meraki/api/#!get-network-ssid-splash-settings
I have some retailers who put all their store APs into a single network. They do this because they want location analytics to default to general reports across their entire brand, rather than per store. With this approach, there is only one splash page configuration. You would only want to use this if you had (say) less than 200 APs.