"Add a + button to the "Library Filter" to allow stacking of criteria in the same way as a Smart Collection."
Probably beyond most users' comprehension! Those who want this type of filtering will use smart collections instead, so it would be better to add an Apply button to the smart collections edit dialog which filters the catalogue without closing the dialog and therefore facilitate building a smart collection.
"Then allow the current search parameters to be saved as a new Smart Collection. "
Agreed + something I've wanted since LR2.0 introduced the filter panel and smart collections.