Lots of people bottle off keg and cap bottles all the time. You can use a counterpressure bottle filler for best results, but lots of people use simpler and cheaper methods with very good success as well. One simple way that I have done it in the past is to use the "cobra-head/picnic" taps. I put a very short (1-2 inches) piece of hose snug over the tap and then insert a bottle filler into the other end of the hose. vent the keg of almost all pressure and apply VERY low gas from CO2 tank - maybe a pound or two and fill bottles as full as you can and cap each one immediately. If it foams over a bit, cap right while the foam is coming out and wash bottle under hot water quick.
I have a medium size tupperware container that I set bottles in while filling and a towel handy - because it is likely you will get some foaming over and that helps contain any mess.
I would think capping would actually hold carbonation better than grolsch bottle, and most competitions specifically state they don't want grolsch style bottles.