What you are trying to accomplish is achievable as I am doing the same thing.
I have setup more than 10 Meraki organizations (another 20 to come) and needed to integrate them with SAML Authentication (ADFS).
The IdP Entity ID should be unique in ADFS, therefore the problem comes when the meraki organizations are located on different shards (different dashboard links). You cannot setup different IdP in ADFS for each organizations.
So, as a king of workarounds, this is the design I made:
1. I have created a Dummy organization set for SAML authentication with a single Dummy network.
2. Gave ReadOnly access to all my Meraki roles to the dummy network. All Meraki Roles are linked to the AD groups with the same name.
3. Configured ADFS IdP Assertion Consumer Service (ACS) URL with the URL from dummy organization.
4. Configured rest of the organizations with the same fingerprint for SAML authentication.
6. Added each needed role in the respective aorganization with needed rights for that organization.
Now, when my user logins via ADFS page, the ADFS will redirect the user to the URL of the Dummy org - but since the role has rights in different organizations, meraki will allow user to select the organization from MSP portal.
This approach works perfect for me since 1 year, but there are few downsides which has to be accepted:
1. The user will see a Dummy organization. That does not impact the functionality, it is just an inconvenience.
2. You should plan your roles very carefully in advance, since one user cannot have more than 1 (one) role assigned. If the user will be part of two AD groups - then only the first group in the list will be sent to meraki.
So, if you want one user to be able to have read only access to one network in one org, and read write access to another network in another org - you cannot define separate roles for both organizations and add the user to both roles. Instead you have to created a common role for both organizations where you will give different permissions in dashboard.
3. The SAML login information will be only visible in the Dummy organization, instead of being visible in organization for which he meant to login. The "change log" will be visible in each organization separately.
4. Camera roles were deployed after my implementation, and strange enough, they don't allow "space" in the name. In the ADFS we have some automation for the role to send - so we have already a filter which includes a space, and I cannot change it - otherwise I have to change the name for 50 AD groups. This might not be the case for you, when you will add each role in Azure manually. We just wanted to avoid touching ADFS every time we want to add new role - since these are two different teams.
But still, if you like to follow some naming concepts, be sure to design the AD groups (role names) without spaces, otherwise you will not be able to use camera roles.
Have fun...
Roman