Well, if you REALLY need to regulate at 0.4Vdc then you could try connecting an Op-Amp do the output and amplify the voltage from 0.4Vdc to say 2.5Vdc which would work with most switchmode regulators. Connect the output of the Op-Amp to the feedback line of your regulator, with appropriate op-amp gain control ofcourse.
I won't presume to know youre requirements, but would you be able to run your system on a linear regulator? Might be hell of alot easier connecting an Op Amp to a linear regulator and then going down to 0.4Vdc.
The support circuitry alone for your switching regulator (plus driving the fets themselves) might draw more power than a linear solution for this relatively low current system.