You can always keep an http handler on the side to do specialized things, but that ties you to asp.net and will prevent your code from being portable on other hosts. If that's something you're ok with, any handler that's registered for a specific route will get executed before openrasta on asp.net.
that said, codecs are the ones writing to the response stream, so provided you have a custom IMediaTypeWriter you can write the resource instance on a stream whichever way you want.
Say for example that you returned an IEnumerable from your handler, as those get deferred executed, you can just start the enumeration of those in your custom codec without any problem.