I just came upon the Supai Adventure Gear Canyon Flatwater 2 from a forum on BPL and thought you may be interested. It weighs 680g, folds up to the size of a Nalgene and has a flexy valve to allow tempering on the go. Obviously not a replacement for Alpacka’s in WW, but I think I might buy one for my partner for faffing around lakes and streams and for my UL river trips.
I am also planning to buy myself one of the stretchy hoses to put on the Alpacka so I can temper on the go. Stemming from this, a good idea to save some weight from the Alpacka’s might be to put the mouth valve on the plug, thereby removing the need for 2 inflation points?