Is it possible to update Ubuntu in a dual boot setup with Windows 7, without booting into Ubuntu?
Not easily, ubuntu is probably installed on an ext3 (or 4) partition and windows can't read that file system without installing some exotic extras, which I've heard don't work very well.
You might be able to do something if you installed Ubuntu via wubi but I don't think it currently has the ability to update yet, only install and uninstall.
The best solution might be to boot into Ubuntu and update it that way, or not update it. If you have no intention of ever booting into ubuntu, then why would you need to update it?