Was thinking about this and was wondering if anyone has tried dropping a small piece of dry ice into the carboy to help prevent oxidation after transferring to secondary/tertiary
Not necassary, typically during the racking process plenty of CO2 escapes from the fermented beer to not only blanket the surface of the beer but push C02 on out of the fermenter. Don't put dry ice in your beer.
Even though I'm about to start kegging and have a 5#CO2 bottle, I picked up one of those at MicroCenter (computer store) for bottling, since I will continue to bottle occasionally. In fact, I just bottled a Guiness Foreign Extra Stout clone today. I'd fill a bottle, sanitize a cap, hold the cap in one hand and the CO2 unit in the other, squirt a shot of CO2 into the head space and quickly set the cap on the bottle. I don't know whether the CO2 leaks out before I get around to crimping the cap on, but it makes me feel better.