need "windowlist" info externally, not interactively

A shell script needs to be able to add a window, and know in advance
what window number is still available.  It will then do some stuffing.

ATM it's quite messy and complex.  The script attempts to randomly
create a title then use that as a handle, but then it must eventually
rename the window to something sensible.  Sometimes it screws up and
clobbers an existing window.

I need a more sensible option.  E.g. I need to be able to do something
like "screen -x user/session -windowlist", and simply get a dump of
windows, which the script can then parse.