The horizontal requirement for water meters is really only for single-jet or multi-jet meters. It's just how the meter works with a spinning carousel that sort of 'floats' in the flow stream there is no rigid axle. The accuracy will suffer and the meter's internal parts will wear more quickly. There are, hjowever, meters that can be installed vertically and even have AWWA and NTEP approval to do so. They are positive displacement meters that use a completely different technology to measure the water. There are 2 main types - nutating disc and oscillating piston.
3 examples that I have used are:
T10 from Neptune
ME-PD from M&E
D10 from Assured Automation
But Bob NH has the right idea about the meter setter, re-setter, or "horn" which are available for purchase but can also be made from a bunch of fittings. This will give more flexibility if needed in very tight spots. just make sure that the meter doesn't require any amount of straight pipe before and after.