Yes, I know for a fact that the 3 in 1s can be set to allow continues (and they usually are), however, I have never seen one that both allows continues and keeps your score. I have played on 4 different ones so far. It may just be a coincidence, so I think we need to look into the issue and see if they can actually even be set to play with continues
and also keeping the score. I'm sure there is a manual for them online somewhere.
If it is found that they only allow continues simultaneous with the scores being set back to zero, then the only issue is really being able to see the total score at the KS. There would maybe still be a fuzzy area for the very low KS scores (they could have died out at 100k and then continued and gotten 800k total, but played at a 900k pace), but cases like this almost seem ridiculous, as someone with actual KS potential would have to be almost purposely playing bad, and then playing good and trying to get a
lower KS score than they can actually probably get. I guess the same holds true for KS scores up to 1.1m (technically speaking, though it gets less and less likely and more and more ridiculous!) Silly tactics, but I guess it would still be a concern that would merit prohibiting them from this list.
I guess I just proved that, either way, they shouldn't be allowed... even though I didn't start out meaning to do so!
QED
Edit: With the exception of what Dave said -if the whole game is documented-
or the settings are documented immediately following the KS to show that continues were not allowed at all.