MIT Commentary Edit
According to Professor Paulo Lozano from MIT in an email commentary "The idea is a very interesting one, and in principle would work. Ion engines have low thrust because you typically have low power available to ionize and accelerate charged particles. You could ionize and accelerate the exhaust of a large rocket if you had a big battery or power supply available. The power levels of big chemical engines are several mega watts, and carrying such a big power supply would be very impractical." A problem that could be suplemented by solar panels on a probe or sattelite. Others solutions could , in theory, pull energy from heat from a rocket blast.
www.aeaeropsace.webs.com - designer of the Hybrid Ion Rocket Engine
http://www.space-travel.com/reports/New_Technology_Hybrid_Ion_Rocket_Engine_999.html - published
Oleg Nizhnik commentary Edit
I doubt it will work for large engines.
4 problems will appear:
1) Braking of gas flow by the bottom grid (may be necessary to use single charged ring at nozzle throat instead of grid)
2) Screening of grid electric field by plasma (hot gas must be non-ionized, very diluted or chamber must be small)
3) Some sort of neutralizer similar to ion engines will be required.
4) Electric power levels required. Doubling thrust of RD-180 by electrostatic grids (assuming it works at all) will require 39 GW of electric power. Most powerfull spacecraft busses have electric power available <15 kW..