Ah! Here's my solution. Repeat the data for as many groups as you have, and then NaN out the data not belonging to each group:
data = rand(100,1);
grpMasks = [(1:100)'<25, (1:100)'<60&(1:100)'>10, (1:100)'>40, rand(100,1)>0.5];
numGrps = size(grpMasks,2);
allData = data * ones(1,numGrps);
allData(~grpMasks) = nan;
boxplot(allData);