I would assume that user simply puts our own URL with parameters into the bang URL template. For example the "Past Week" filter appends &dr=2, so in the bang template you can just put https://kagi.com/search?q=%s&dr=2 to make a "Past Week" bang. Admittedly that requires a few exploratory searches to figure out the parameters (I could take a look at adding that to the docs), but does that satisfy your request?
