Mine gets bitten really badly there, I started slathering sudocreme all over her inner thighs and teats, but it melted off so I have started using that Lincoln 'antibacterial green gel' which is one of those horrible greasy sticky substances which sticks like glue and is therefore awesome for putting on the teats and all the way up her navel as she gets bitten there and rubs it. The skin feels much less scabby under there now and it contains deet to keep the flies off. Nightmare washing it out of her tail mind!
She also has an ordinary fly rug for bad days which does help but you can get an udder cover from snuggy hoods!
I'm following this thread with interest: my loan mare gets really stressy because of flies, poor old lass, and so in desperation I put her in a sweet-itch rug with a long neck, however she still gets flies in all the wrong places! bless 'er.