I can’t really come up with any argument that would be against this proposal. The only one I see is that I can’t think of any use case for this and it would be confusing to read for nearly everyone.
Also do you have any link that explains how those numbers work exactly? My guess is that it is something like
hexToDecimal(0xA.BpC) = (hexToDecimal(A) + (hexToDecimal(B) / 16^hexDigitCount(B))) * 2 ^ C
with A, B and C as hexadecimal numbers
In the end you should probably create a feature request at https://kotl.in/issue with an explanation of how the numbers work and as many usecases as you know of. That said, unless you have a very good usecase I don’t think this will be a priority and you might need to wait quite some time for this to be implemented in kotlin (if ever).
Also if you are confident enough you can try to write a compiler plugin that allows you to use such numbers. Even though those are still experimental and undocumented there are a number of talks/videos on how to use them (I think they will be released in a more official if still experimental form with kotlin 1.5). This might also help in getting this added to kotlin as a default.