Ethan
Many years riding a Hase Delta in the UK , always though I would one day tackle one of Brads.
Yes I would be tempted to make it 2wd , there is a condition the Hase suffered from rarely where when riding up a steep hill the lightly loaded from wheel would hop across the path away from the driven wheel.
In the UK this was relatively benign as the hopping took you away from any road traffic , that would not happen in some other countries..
So to achieve 2 wheel drive you need an additional drive shaft between the wheels and the pedals.[ called either jackshaft or layshaft ]
Take the existing rear wheel shafts and add 2 free wheels and don't add the cassette.
The new shaft runs in similar bearings to the wheels [ although you may get away with only 2 sets ? ]
On the shaft is a free wheel for each rear wheel and between them in line with the pedals is the cassette.
If you can get the angles and distances correct this can have the benefit of completely replacing the pulley under the seat for chain management .
The other way is probably harder to engineer but looks neater with less chains etc.
The 2 shafts in Brad's design terminate in the middle of the trike on a pair of freewheels , now imagine the free wheel as a pair of plates the outer plate is connected to the each rear wheel , the inner plate is connected to the other inner plate with a single gear wheel sandwiched between them and the free wheel mechanism allows drive between each pair of plates as per a normal bicycle.
So the chain drives the centre gear wheel and if conditions are right both rear wheels turn
On initial assessment this is not good as you only have 1 gear ?
However it then gives you the ability to have either a IG hub or a cluster driven from the pedals and then on to this single gear.
I can draw the first example and have a coloured drawing from the old thread of the centre of the second example.
I believe the old thread can be found on the Internet Archive = Wayback machine.
try this , it may help muddy the waters ?
Free wheel diffs
Someone cleverer than me will be along shorty to explain this better.
If there is any of it you don't understand please ask , it was not my intention to confuse you .....
Paul