I was once told that if I submitted my orders at least 2-3 minutes before the market opens, it would participate in the open. FOK specification was available at Ameritrade. Izone stated offering TradeTrigger since last October. Not sure if it still would be better than just2trade or not.
A FOK order placed 2 to 3 minutes before the open is not the same thing as an "opening only" Limit-on-open order. Trade triggers won't help you either if you want to place a true Limit-on-Open order. A "market-on-open" could be achieved by placing a regular DAY market order up to a few minutes before the open. If anyone knows otherwise i would love to know about it.