Brandon, You could certainly go ahead with your project without a ground plane and see if the lamps fire up reliably. The ground plane is used to reduce the voltage required to fire across the length of the lamp. The ground plane acts as a capacitor, allowing the initial arc to fire between one of the ends and the nearby metal plane. The arc, once started, can then sweep across the bulb until the entire bulb is lit, thereby eliminating the need to fire across the entire bulb. It is reported that rapid start ballasts need this more than other ballast types as they rely less on higher voltages.
The down side to having no ground plane, especially in rapid start applications, is that if the ballasts have a hard time firing the bulbs the life expectancy of the bulbs is said to be shortened. Since you are using an instant start ballast, which does not heat up the filaments in the bulbs, this should not be a big factor so long as the ballast is able to light the bulbs. However, another use of the ground plane is to reduce bulb flicker. Who knows what effect bulb flicker will have on the UV emission of your bulbs, which is what you are after. At worst, it will probably just make your process timing a bit more difficult, but who knows. If you try asking the bulb manufacturer all they will tell you is to construct it properly and install the ground plane.