Editing the $RET value using the weblog_standalone hooks

I’m looking to be able to specify a ‘return=’ parameter using an extension. I’d like it to be applicable to weblog_standalone and the comments module. I’ve already worked out a simple hack for the module files, but it’d be nice to make an extension that did the same thing. Any help in getting me to understand how the extensions intervene in that process would be great. Thanks.