The injection pump on the GM 6.2/6.5 diesels sits in the valley, smack in the middle of the engine. It gets hot anyway. I don't think heating the fuel/oil to coolant temperature before hitting the IP will cause it ANY harm. I have about 25k miles of WVO (205,000 miles total) on a '95 Suburban without (known) issues. The rotary IP is not very robust to begin with, and with all the sand, dirt, and dust in the area, I believe fuel contamination, and poor fuel lubricity is more likely to be causing failures than heat. NOT heating the fuel prior to the IP is more likely on these to cause issues. I've seen reports on GM diesel forums of unheated fuel being too thick contributing to pump shaft failure. On my setup, both fuels (diesel and WVO) go through a flat plate heat exchanger prior to the IP. That way, when I switch fuels, there is no thermal shock.
FWIW, YMMV