Yes it can be done with the use of easybcd neogrub and 7 zip.
Install 7-zip and neogrub to windows 7.
Create C:\slack directory.
Extract bzImage and initrd.img from iso using 7-zip
Move or copy bzImage, initrd.img, and slackware iso to C:\slack
Use easybcd to install neogrub as a boot windows boot option
Edit C:\nst\menu.lst with following lines
at the windows boot menu select neogrub and you should get a menu to boot slackiso.
I can't remember if there is an option to select an iso file at the source section of setup.
If not you can create an a new directory and mount the iso there before running setup, then when asked select premounted directory.