I found a way to do this, but it's not the best. What seems to work is to have the variables presented at the beginning, have the experimenter enter a comma for each one, and then go to the instructions, etc., and have participants enter new values for the variables. The commas present as blanks, so participants only see what they have entered.
One problem is that since you're presenting variables twice, MediaLab pops up a warning every time you run it. Also, it relies on the experimenter to do everything right. It leaves you with two sets of variables in your data file, one that's all missing data, and the other one is what you care about. So, it works, but I'd still love to hear if there is a more elegant solution!