You could try setting up on-net web filtering. Within an EMS Profile you can define the 'work' networks, then under the webfilter section of a profile enable "Client Web Filtering when on-net". There are a few gotchas here, one being web filtering will be off when the user isn't in the office.
Personally I'd whitelist (in the Web Filtering part of the profile setup) the Splash page. The Exclusion List setup is very flexible so there are most likely multiple ways to achieve what you need. Do you know what the splash page URL is?