Note that I cannot think of any portrayal in print or art from history or recreations that shows any of these pre-modern weapons being used by someone who is prone (exception: blowgun used by warrior lying flat on a branch, but this is as much for stealth as aiming and might be a movie thing anyway). Any advantage should be offset with penalties. Also note that firing from a lower position reduces the range of the missile (more significant with shorter ranged weapons). You also run the risk of obstacles, even simply foot-high grasses, obscuring the target.
According to some depictions, Chinese crossbowmen using mass-firing techniques fired while supine, but that does not allow for precise aiming. Depending on crossbow design, this can improve leverage for reloading.
Kneeling is shown in several sources, including medieval illustrations of the crossbow, but always behind a defensive barrier (where loss of time to reload and get back into firing position is not an issue).
I would be happy assuming that any appropriate steadying of the weapon is assumed in the weapon tables and improved OB comes partly from better skill in the necessary techniques. I can see granting a small firing bonus while kneeling or prone (and more importantly, I'd grant prone a *bonus* rather than a malus against incoming missiles, or at least not make a prone target easier to hit). You can also gain a bonus to Hiding for ambushes, but reloading should require changing position and the loss of time should be sufficient that normal usage penalizes rather than rewards the firer for getting into these positions. Otherwise, one would have expected them to be standard methods. I would prevent anyone kneeling or prone to shoot from using DB from Quickness, since he is trying to minimize movement.
