Some of the Saleae logic analyzers do support selectable thresholds. Full details on the threshold options for each product can be found here:
However, none of the products support a continuously variable user selectable threshold voltage. Specifically, that means that for devices with selectable thresholds, only a few fixed options can be chosen from. The user cannot choose a threshold voltage outside of the provided list of options.
We are keeping this in mind for future product generations, most likely by feeding the input comparators with a DAC.