Pulling the code from Entlib 4.1, and updating it 5.0, is pretty much your only option short of rewriting it from scratch. Getting the code running shouldn't be hard.
The security concerns were around leaking information between users, so if that's not an issue for your app, it's not an issue for your app. It was way too small a "well, it's ok in this one tiny circumstance" to get it past Microsoft security
The other major issue was the use of the ASP.NET cache as the storage mechanism. Turns out I was wrong and we found out that the ASP.NET cache doesn't scavenge properly in non-web applications. But again, you're in a web app already so that should work fine.
The other major problem is that there's no good way to determine a unique key for the combination of inputs. As a result, there is a non-zero probability of collision between different sets of inputs, and there's no way to tell if you're getting a false
hit in the cache.
Evaluate these factors, decide for your application, and move forward.