You can try this for now, if you want and if you have a fw 100a.
Thanks blackhole. I'm happy to report success. I will post more details as I have time to test all the functions of CHDK.
For now I can report:
I believe I do have firmware 100A. ver.req doesn't seem to work with this camera as expected with newer cameras. CameraVersion.exe tool does not recognize images taken by the camera with the message "Unknown (0x3890000), but it does state "version" as "100 A". This is on a Canon factory refurbished camera bought on Amazon January 2017.
I copied the files to a 2GB SD card formatted as FAT32 in Windows 7 Pro using Disk Managment run as administrator. Because the firmware update method of booting CHDK didn't work (see below) I used EOScard 1.40 to make the SD bootable.
CHDK boots when accessed through the "Play Mode" button.
CHDK will run scripts, specifically tested was the "intervalometer.bas"
CHDK will create DNG raw files in addition to JPEG. The sensor in the ELPH 170 actually seems to be reasonably low nose when compared to the extremely aggressive noise reduction applied in camera to the JPEGs. I had good results converting the DNG to TIFF in Photoshop CS5 and UFraw 0.19.2 on Windows 7 and 8.1 respectively.
CHDK menus appear to work fine.
Alt/Play button works fine.
Shutter, zoom (digital/optical), "?"/help button, Func/Set, Disp, Eco, Auto/P mode toggle and flash toggle all work as expected.
Firmware update using a non-bootable SD card and the camera's firmware update option in "Play Mode". Update firmware dialog on the camera shows with the text "Update firmware version? 220.127.116.11 --> 18.104.22.168" but selecting "OK" causes the camera to just power off unexpectedly with no LEDs or other dialogs.
The power on button does nothing. Power off works fine (the same button). Entering "Play Mode" when the power is off loads CHDK. But attempting to power on the camera and load CHDK via the power on button does nothing - no LEDs, not power on, no lens extension. (NOTE: pushing the shutter button after powering on the camera with CHDK via the "Play Mode" button extends the lens and enters the default record mode function, this is the same result as the power on button working.)
Thanks to all the CHDK developers for this awesome software, and especially to Hardware_Hacker and blackhole,. I will update this post with further tests of all CHDK functions I am familiar with as I use the camera.