Andrew,
Pretty much, I am allowing you to specify the order or all filters, including rp_filter, which is what you refer to as "hijacking" filters. You can register any filters you like and of course, you can hack them to run before rp_filter by naming your package someting that comes before acs-tcl.
I am just making it simpler by giving rp_filter an explicit priority, which you can override by making your filter a higher priority.
In general one should understand that you are also overriding the built it, well-tested, mature security code in the request processor when you do that.