I think you better check with Bilstein before you do this. A nitrogen charged shock mounted upside down allows the nitrogen to rise to the top, therefore the piston is cycling through nitrogen instead of fluid. The nitrogen is there to pressurize the fluid (oil) to lower the temp that the fluid aerates.
Not to mention the fact that if your reservoirs are on the bottom they're more susceptible to being smashed by trail hazards.
I hope you're not mounting them upside down just so everyone can see them?