I'd just like to step outside the space of a specific solution for your problem and address the underlying desire you have to consolidate your workload.
Be aware that any patching/upgrading scenario will have costs that you must address - there is no magic pill.
Particularly, what you want to achieve will typically incur either a build/deploy overhead (as jonnii has outlined), or a runtime overhead (in validating the new versions to ensure everything works as expected).
In your case, because you have already built your products, I expect you will go the build/deploy route.
Just remember that even with binary equivalence (everything compiles, and unit tests pass), there is still the risk that the application will behave somehow differently after an upgrade, so you will not be able to avoid at least some rudimentary testing across all of your applications (the GAC approach is particularly vulnerable to this risk).
You might find it easier to accept that just because you have built a new version of a binary, doesn't mean that it should be rolled out to all web applications, even ones that are already functioning correctly (if something ain't broke...).
If that is acceptable, then you will reduce your workload by only incurring resource expense on testing applications that actually need to be touched.