Fancier feed manipulation in aggregator module

The aggregator module lets you use call:function syntax in the aggregate setup to modify the text from the feed, but it's only set up to do one call, it can't nest them. I tried to modify it to nest the calls, but it didn't work at all.

I was thinking of either extending aggregator, or creating a new module which lets the user create anonymous functions using create_function that can be accessed from Module::function

Given Aggregator is part of a default install, is it best to do a separate module? Or given Aggregator is the only module likely to use such a feature, should I combine it?