I haven't tested it myself, but since the internal captive portal (splash page) can be set upto 90 days, and you are only changing the location of the portal but not really dashboard behavior, its still the Meraki cloud platform that grants access on the AP and redirects the user to continue to the URL, I would try test with 90 day duration and beyond ( I have a good feeling you can do 90 days not a year).
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