Is there any way to control the white toner without using a rip program?
There is an involved process in Photoshop that will allow you to screen the print (which, by definition, reduces the amount of all toner).
I've been using the 920WT for several years now and know of no method of actually controlling the white toner coverage. I guess you could play around with the density settings?
That all said, while the initial outlay for the RIP software is large, I've also been using Transferrip from Forever for several years. I initially considered it to be a reduction of $1 profit per shirt until it was paid. Using that line of thought, I had it paid for in short order.
Since then, the reduction in toner use, the color control it provides, and most importantly, the end result of the much softer hand and higher durability makes the outlay more than reasonable.