Is there some way of telling startcases/stopcases to behave in display mode?

Yup, I just changed a couple of things and now we have a \startbigcases ... \stopbigcases for this. Will upload new version soon.

Oh, great! I'm really looking forward to get this new version. By the way, would you be so kind to include some multiline equation environment? Something like (or better than) \[ first part of very long equation \eqbreak second part of very long equation \eqbreak third part of very long equation \] which I achieve at the moment by doing \[ \startalign first part of very long equation \hskip 1.5em \\ \hfill second part of very long equation \\ \hfill third and part of very long equation \stopalign \] But this does not work as well I wish. I'd like to substitute the '\hskip 1.5 em' after the first line for a '\hfill' so that it always be the leftmost line, but the latter does nothing at all. So I have to put a '\hskip' and tweak the distance by hand. Thanks for you help, David