Check this out:
x = (1:5)';
freq = [.4 .3 0 .2 .1]';
ecdf(x,'freq',freq)
You can provide x values and their frequencies, and read the desired quantiles off the graph. If you really want to compute them directly instead of reading them from the graph, you could try the two-output form:
[a,b] = ecdf(x,'freq',freq)
This returns the graphed data. You could use these to compute what you want. I can imagine counting the number of b values less than each desired quantile, and taking the corresponding a value. But maybe someone has a more efficient way to do that.
