Relax, you're not the first person to run into this problem. Folks here have offered you solutions. You can take 'em or leave 'em, it's up to you. But in the grand scheme of things, this seems very minor.
You are correct and I am grateful for the info you supplied. The reason I was surprized about this 6x7 issue was because I thought 6x7 is a standard format and couldn't believe my eyes when I read that the Nikon Coolscan 9000 only scans strips of up to 2 frames. My surprize got even greater when I read that the Epson V700/750 covers the full 8x10" area, yet only scans 2 6x7 frames at once. I mean, come on, flipping over the film holder certainly is a work around, but what exactly were the designers of this film holder thinking?
Just for comparison: I have scanned dozens of 135 films with my Coolscan V. The negative bags hold strips of 6 frames in one row, and that's exactly what the scanner can do in one pass. With the film feeder that came with the standard package!
Who's cutting film into 3+3+2+2? I always have my film developed with instruction not to cut. I pick up the roll and cut it myself. Then I can go 3+3+3+1, or any variation thereof.
I develop my slides at home, so nobody cuts my film against my will
For aestetic reasons I prefer 3+3+2+2. If I follow your suggestion of flipping the film holder, 3+3+2+2 makes a lot more sense, since I could scan all 10 slide frames in 3 scan passes (vs. 4 passes for 3+3+3+1)
Have you looked at the film holders from Betterscanning? They used to have a wet/dry mount holder which used pre-cut template for various film types. There may be one for 3 frame strips of 6x7. I honestly don't know.
I just checked them out. The max film strip length their holder allows to be scanned in one pass is 213 mm, which is short of the 230mm my film strips with 3 frames have.
What's so hard about making your own film holder?
In my long engineering career I have made the observation that quite often, when commercial products seem to impose a random limit, there is indeed a non obvious but valid reason for imposing this limit. So I see a scanner which can scan 8x10" slides, yet all the film holders made for this scanner for some unknown reason stop at 213mm film strip length. I don't have this scanner yet, but would love to know what I'm up against if I decide to get the V700. If this modification really works and I can indeed scan 2x3 frames at once after removing some plastic from the folder, I'd be very puzzled. After spending 500+ on a scanner one shouldn't have to do that himself.