I haven't done what you are asking, but I don't see it being an issue.
I have done something that might be worth considering for your project.
For migrating firewall rules sets from other firewalls to Meraki, I wrote a tool that can import a ruleset originally created in a spreadsheet.
https://www.ifm.net.nz/cookbooks/mfw.html
It supports creating objects. Meraki (at least at the time) didn't support objects or object groups. My tool allows you to both import and export the rules. To keep the linkage between the object names and definitions I make use the of the "notes" fields to store the original object names. I create a coma separated list of variables in the form name1=value,name2=value, etc.
What you could do is create an additional network under each customer called "metadata" (or anything you want) and then make use of the network notes field in the same way. This is extendable because you can store as many variables as you like in the same way as I do. You could probably even store a json object.
This network would not contain any inventory. It would only be used for storing your metadata.