Not that I know of. In the end you are editing the browser rendering parameters. Anyone can inspect the page and see that the opacity on the page is being turned down. Finding where it is happening is the only thing you can really make hard. Have a couple of the pass through scripts be machine generated and you can have it use nonsensical variable names and a bunch of dummies that lead on wild goose chases. It could all be fixable, but you can make it a pain in the ass. Add a redundancy or two and it will make debugging a nightmare because even if one is fixed, the others will make it look as though it has not.

The real answer is to have NEVER do freelance web development inside the client's firewall. Never. If they try to require it, walk away. If it is inside their firewall then they can just take the source code and stiff you. If they try to spout some BS about security, say that is precisely what you are concerned about and point blank ask them what safeguards they are willing to allow you to put in place for developing in their system. If the answer is none, walk. If they are willing to let you VPN in, run the code from a local copy over the VPN and node lock it so if someone attempts to serve it from another machine it fails.

Apologies. I'm tired and hate businesses taking advantage of "Independent Contractors".