Oh, maybe a 10 foot cube filled with line...
And that's "absolutely trivial"?
My understanding of delay lines is that they tend to add a fair amount of "fuzziness" to the signal going through. If the signal is only being delayed by a few hundred nanoseconds, it can remain reasonably clear, but delaying a signal for tens of microseconds while retaining 100ns accuity would seem difficult. I know storage tubes could almost certainly do it, but those would seem to be overkill in a consumer-grade television set.
The only thing I can really imagine could have been done differently with a color standard while remaining easy to decode would have been to do something like what VHS does, amplitude-modulating R-Y and B-Y at different carrier frequencies. This would have had some advantages, I guess, but I think bandwidth issues would be a problem.