Searching splunkbase (splunkbase.splunk.com) for "meraki" returns six results. Perhaps one of them supports syslog.
You'll have to ask Cisco about Meraki support for JSON.
The Cisco guy told us -
syslog export in JSON format is not a viable request. All the syslog data is sent in arrays. This is in accordance to RFC5424 export format standard for syslog.