This feature request is basically about implementing metric based altitudes in IF in ATC instructions. A few countries in Asia use metric altitudes below RVSM, and China uses metric-based altitudes even in RVSM. The implementation is straight forward - just add them to the list of altitudes when in the metric airspace. Pilots will still fly them in feet.
There will be some issues related to compatibility with the current separation rules. Due to rounding, some lower levels will be separated only by 900ft. These can be worked around by reducing the separation requirement within metric airspace to 900ft. Another issue is inactive aircraft causing metric and imperial altitudes to mix (until climb VNAV is implemented at least). Between 290 and 411, this can be solved by treating 290 and 291, 300 and 301, etc. as the same altitude. Higher altitudes are pretty similar (430 is in both metric and imperial, treat 449 and 450, 469 and 470, etc. as the same). Lower altitudes are more complicated however, because the next altitude below 291 is 276 in metric. There could be a rule that an aircraft needs to be active when entering/leaving metric airspace below FL290 to avoid this problem (or alternatively, allow an AFK VNAV descent).
Another question is whether metric or feet based messages should be sent in ATC. Metric messages are more realistic, but they can also get confusing. One solution is to put the equivalent feet FL in the ATC menu.