Snippets from the documentation on Read-Only Mode Overview:
When your organization is in read-only mode, Salesforce Classic desktop and mobile browser users will see a banner at the top of their browser window.
Users attempting to edit records during read-only mode in Lightning Experience will not see a banner, but will receive a “Review below errors” message at the top of the record upon attempt to save any changes made.
If you are accessing Salesforce via the Salesforce mobile app, upon logging in you will be brought to a page notifying you that you are in read-only mode; there will not be a banner at the top of the page.
In the UI, the error message is, "Updates can’t be made during maintenance. Try again when maintenance is complete."
In the API, the API error code is INSERT_UPDATE_DELETE_NOT_ALLOWED_DURING_MAINTENANCE.
In Apex, the exception will be of type InvalidReadOnlyUserDmlException.
My guess, since its the mobile app we are talking about, is that the record will get queued on the phone (if it doesn't have internet connection at the time), and then it will error on all the records that it tries to insert during the read-only maintenance window. After the window is closed, your users will probably be able to "retry" inserting the records.