I'm fairly sure you can do it by putting the rndis.rsc file in \flashfs\system\appcore.d\factory.d
This file is in one of the early hack zips.
with rndis the only thing you can't do is webcam mode - you can pull your saved images via the Flir toolbar thing, or FTP
There is a
zlast_RNDIS.rsc in the firmware file for the FLIR iX series firmware. Could you just rename that file, or is the real RNDIS.rsc different?
Attatched are .rsc files for normal and rndis.
I'm not sure what happens if you have both - my best guess is they are read alphabetically, so to do rndis without touching the existing usb.rsc you may need to rename it to something later alphabetically.
(This may explain the zlast name above)
The files are CR32'd, so can't just be edited without re-CRCing (method a few pages earlier here)
I did try making one that did all 3 protocols but it went a bit nuts..
BTW a reminder that the first thing you should probably to if playing is run RNDIS and use filezilla to ftp off a backup copy of the whole \flashfs structure, as this is all the data specific to your camera.