The Mill is a commercial venture and so what we provide is driven by the user community in the form of programming languages and other standards; our job is how we provide it. The UNUM proposal essentially represents the exponent itself in floating point so that (for common values) the significance is improved, to the point of exactness in many computations. This is incompatible with the standard IEEE representation, so adopting it would require changes to language, standards, and much software as well as the hardware, even it it were only an extension and not a replacement to IEEE754.
I am not enough of a numerics guy to judge the merits of UNUM on mathematical grounds; my role on the 754 committee was as an implementation gadfly, not an algorithms specialist. The small examples of UNUM usage provided seemed to work well, and the implementation in hardware would be straightforward, but I don’t know enough to judge its merits in general code. My gut feel is that hardware prices and operating costs don’t warrant another format when one could simply do everything in Decimal or quad Binary precision. The time for UNUM to have been introduces was back in the day of the original 754, 40-odd years ago, when there were many incompatible formats, and a good idea did not have to surpass embedded practice. That chance is gone, perhaps unfortunately.
There exists a standard that tries (in a different way) to preserve precision, the IEEE Decimal standard, which the Mill supports. If UNUM reaches even minimal acceptance then it could also be incorporated in the Mill, and likely would be. Until then, even if the hardware had support there would be no way for you to access that hardware due to absence of support in programming languages.
An initial implementation of UNUM would be more suitable on a register machine that can be usefully programmed in assembler, even without HLL support for the format; the Mill is not a realistic assembler target.