Harbin is another pear rootstock that is rarely used but the defunct Bear Creek nursery used to use for Asian pears because of it’s high level of cold tolerance. I have a nice Korean Giant I ordered from them over 25 years ago on it and it is the nicest Asian pear tree in form and vigor I have anywhere.
Betula is my second pick for A pear roostock after Harbin. I’ve heard from nurserymen that there is a wide range of hardiness in various strains of Bet. but trees I order on it have never suffered from what we get here. Certainly its been fine down to -17F or so on the strain ACN uses.
Asian pears are dwarf enough without slowing them down with other pear rootsocks (besides the cold susceptible Cal) unless you want a full dwarf tree. I speak from some experience with A. pears on ohXn 97 trees. They take too long to establish as well. FNand B Inventory says A. pears on E.pear rootstocks are relatively short lived.