Just about any bearing's seal will hold with oil, not because of the seal, but because of the forces involved. Seals are (almost) always fixed to the outer race with the seal lip on the axle or inner race. As the bearing spins, oil will be flung toward the outer race, and at even moderate speeds spread itself around the periphery. When not in motion the oil will settle to the bottom half of the bearing.

So, as long as the oil level is not up to the axle or inner seal, the lower third (or so) of the bearing will act as an oil sump, and there won't be leakage. Of course, there might be some weepage if the bearing is turned on it's side, but if the seal is intact, surface tension at the gap should prevent any loss.

IMO, the ideal arrangement for this kind of lubrication would be bearings with outer seals only, and a hole drilled in the middle of the hub shell, or other method, so some new oil could be added as needed. Kind of the way bicycle hubs have been made for over a century.