Use only the straddle cables that come with the brakes or the correct replacements that we, and other companies, sell. I know that Dia Compe makes some longer cables, but those are intended for other models of brakes, not for the 610. I just spoke with the president of Dia Compe and he says that they plan change the cable attachment so that future versions will work with longer straddle cables. There are a few workarounds if you really can't afford a new rear brake for you mixte, but I'm not satisfied that they work well enough.
By the way, these are super brakes that I use myself (with the proper cable).
Also, a big thanks to Aaron of Aaron's Bicycle Repair in Seattle for bringing this issue to everyone's attention.