First, my apologies for the long delay in answering. The idea behind this patch, and the patch itself, are very interesting. I'll look into it for Wget 1.8 (1.7.1 should be a bugfix-only release.)
Several random musings: * It would be nice to have an option to use only one filter, so that people who want speed and/or retain state between filter invocations get get them. * \ is probably not the best choice for the escape character; it's easy to lose it. Maybe %u et al. would be a better choice?
