Edit: As xopher brought to my attention, the image dimensions used below pertain for unscaled streaming (eg to the n900's screen). Using phonestreamer will require different values related to te image dimension being streamed. BUT the principle still applies.

V4l2-ctl in fact in deals with digital "zoom" in particular. You can try it out as follows.

Needed: working ssh access to the n900 and phonestreamer, or

stream to phone screen: and v4l2-ctl on the n900 (make sure it is executable): http://talk.maemo.org/showpost.php?p=501105&postcount=7

Get the streaming going. Now, either in an x-terminal on the n900 (if streaming to n900 screen) or via ssh (best from the pc being streamed to) and making sure you get the correct path for the v4l2-ctl exe:
This should result in a "zoomed" stream
A "zoom-out" effect.

By fiddling with the top/left and width/height combos one determines the location and extent of zoom-in on the original image dimension. The effects here are entirely digital as the n900 lacks an optical zoom. If you just run v4l2-ctl you will get all the possible parameters. So yes, quite easily phonestreamer could zoom the stream. Best however to talk directly to the camera API.