RD, I have not yet seen 250R specific grab rails. When my girlie rides on the back, she usually sits comfortably straight and just keeps her hands on my hips. Most of the time she's not even holding on, and they're resting on her knees, and her and I have done multi-hour, multi-hundred mile day trips like that before. If she starts to get uncomfortable she jabs me in the ribs and I pull over so we can both stretch. Sometimes when she gets tired she'll actually put her helmet on my back and just lean on me - she fell asleep like that once... Last time that happened.
If she doesn't like squeezing you the whole way, she could also put her hands on the gas tank and lean on her arms for a while to stretch out the lower back, if her arms are long enough.
The grab rails Blue Ghost posted are also great options!