Solution A: Wooden pipe with redstone engine on the condenser, advanced insertion pipe below the geogen, putting overflow in a "waiting circle" of stone pipes. Disadvantage: Will not drop the lava cells, but may lag anyway. The pipe will flood, since the redstone engine pumps 10 times more cells the generator burns.
Solution B: RedPower filter requesting one cell, toggled by a 10sec timer. pneumatic tubes below the geogen as well.
Solution C: provider logistic pipe on the box, supply logistic pipe below the geogen, set to request a stack of cells, enable partial requests. Disadvantage: expensive. Advantage: very flexible when adding more generators, will not produce any spill at all.