[fieldnstream]

My dad is a bonsai guy and has some ficus sp. that he's been working with (especially F. microcarpa var Green Island). I know that he was working on forcing some roots...I'll see if he has any tips. I know this isn't directly related to your question, but have you tried training them by slowly removing soil? That should give you plenty of options for selecting which roots to keep and which to remove...but wouldn't help much if you are working with an established plant.

 

[fieldnstream]

To get the best control of root placement, you could slip-graft them. It would be easiest if you have a couple of extra plants to take stock from.

 

[fieldnstream]

Like upside-down t grafting with a straight scion...I think there are other names for it (probably more commonly used ones). Does that make sense?
EDIT: My dad called it slip grafting...he is a neologism factory though.