I guess that this is the simplest solution:
Go to:
- Administration -> Profiles
- Create new profile ( named like 'Visitor')
Then:
- Administration -> Visitor
- ( For your request ) Under Precerences -> Maintenance add the flag for View, Add ( maybe also for Edit it's not a problem ).
- Give all the other permissions that you want to give to this profile
Create an account with this profile and give it to your client, when his IP changes, he must add his IP in the backoffice, with this profile he can't make damage. ( Obviously, your clients have to know the URL to administration panel )
Otherwise your clients must to ask to you every times to add their IP in the maintenance field.