This is a very common thing with some oak species, particularly "live" oak species. Lower branches in very old specimen of Quercus Virginiana (southern live oak) droop low and root at the ends.
Although it's a very compelling trait, it's not easily used in bonsai. Finding appropriate stock is the main problem. Such trees in nature inevitably have a low broom style--accompanied by a very large initial trunk. That can take some time to find in an containerized oak.
Which means finding another species and using it to develop this style. Ficus would be the most obvious choice...
Also, once developed, a container to hold the finished tree might be problematic, since you have to provide a pot wide enough to accommodate the rooted branches...