I tweaked the cheapo filter capability so it operates at the level of the
overall index. If you apply a filter using the form in the left-hand sidebar, say, "Bottechia", it will only display those months in which a message subject mentioned that brand (or someone with "Bottechia" in their email address posted). That filter will continue to be applied until cleared, so if you poke the "dates" or "threads" link it will apply. Furthermore, if you poke the left or right arrow links on such pages, it should take you to the previous or next month with messages matching that filter.
This still needs a bit of work. I'm happy to field questions or reports about problems with this feature.