Of course they wouldn’t work, as you merely have copied the files. But (most) video-DVDs need decryption and all that fuss, too.
Why the images work in other players or programs is beyond me, they really shouldn’t work there.
Or were you using AnyDVD alongside?
And why were you ripping DVDs with Clone CD? 
CloneCD probably only does them in ISO9660, whereas video-DVDs need to be in UDF 1.02. Maybe that’s why they don’t work.
" and re-ripping would be a real pain."
If it’s really just because of the way CloneCD has made the ISO, there’s no need to rerip.
1.) Mount the ISOs with Daemon-Tools or a similar program.
2.) Burn the VIDEO_TS folder with a burning-program (ImgBurn, Nero etc) as a DVD-VIDEO (important!) to a new ISO-image.
3.) Success.
