In my honest opinion, this would be difficult if not impossible...The form 'banyan' is not a form developed by artists which are governed by "rules", but rather by studying various natural tree forms...To best study this form I would recommend the study of trees in nature which naturally grow in this form...
Fortunately there are many examples of banyans with aerial roots here in Florida, but aside from aerial roots are there any kind of style guidelines or rules that are supposed to be followed for branching and shape in the canopy?
Taiwanese Banyan styled trees -mainly ficus- are the best examples of fine bonsai cultivated in that form.(Although they rarely show aerial roots,personal preference I believe).I like to study these trees very much.
Banyan trees basically follow the same"rules" as conventionally grown upright decidious trees in the natural style,however their ratio of hight to width is usually reciprocal but not limited to be.There is a basic limb structure with a nearly exaggerated extended first branch;twigginess is achieved with considerate controlled upgrowth to form the typical Banyan shape.
The tree will develop-hopefully-aerial roots,which become multiple trunks and eventually melt into each other adding steadily to the girth of the trunk.Sometimes one have to eliminate and/or straighten aerial roots.
The best ficus (!) material for the Banyan style in my eyes is a tree that readily develops aerial roots (even the same variety can extremely differ when it comes to aerial roots)and shows little "melting plates" on the trunk,places where aerial roots have already melted smoothly into the trunk.
One of the biggest mistakes I notice with Banyan style grown trees is the uncontrolled growth of aerial roots and neglected surface root development and correction.