actually no you can not contract genital herpes that way!!!
if you have hsv1 orally, for starters, that gives you significant protection against contracting hsv1 on other body parts. It's not 100% but it's up there. also saliva is a very poor way to transmit all std's in general, including herpes.
is it a good idea to use saliva as a lube for masturbation in general? No it is not. it's not a good lube and it also increases your risk of genital yeast /fungal infections. buy yourself a good sex lube like KY or astroglide for masturbation. They work well to protect your delicate skin during the friction of masturbation as well as are designed to not increase issues like fungal infections.
grace