I don't think this would be advantageous to have in the language. Tuples are not well-suited for storing large amounts of data, so a syntax making that easier feels undesirable.
In my experience, I rarely need to write comma-separated lists with repeated values and non-repeated values interspersed with each other, so I don’t think I'd find this syntax much more useful than a repetition-only syntax.
If this is something we want to sugar, I feel it should be done with the repeat keyword, since that’s the keyword variadic generics uses for splatting.
I don't see why we should reuse the InlineArray syntax for non-empty Arrays. [some of some T] would still be possible if inline arrays used x.