Harm Paulsen told me, that those bow can be strung by standing the lower end on the ground (he uses a patch of fur under the tip to protect it), pull down the deflexed tip with your whole body weight and string it. The backward bend keeps the levering force behind the belly, so you don’t need a knee or thigh as a hypomochlion. That only works for straight bows, of course. If the limbs have a natural reflex, then you would still need to string it the usual way, I guess...