UC Irvine's Expectation-Driven Event Monitoring dynamically composes filters fetched from the Web. Imagine extending a flight-reservation systems' command interface to interpose a graphical map. Apple Computer's Web Objects demonstrates an airline UI prototype. The challenge is that only first-party stakeholders can extend their software architectures in these ways today. However, we can envision neater, more efficient ways to enable reuse.
PEP also affords reasoning about compatible extensions and composite extensions.
We are already familiar with many analogues to active proxies as reusable filters. The difference is in the the affordances of the interchange format. Most significantly, since PEP modules are identified by the URI of the protocol they implement, PEP-aware Web tools can negotiate common sets of compatible modules and settings. PEP enshrines a philosophy of decentralization. Anyone can publish an extension by maintaining a Web page describing it.