It's been rumored for a handful of years that Hetfield has had something in the works, but I don't know that they'd ever release anything. Maybe, maybe not. It would have made sense in the mid to late 90's at the height of their popularity, but I'm not sure about now.
The last album they released was 8 years ago, and what...2 albums of new material in what's close to 20 years? If you consider Load/Reload as 1 album worth (all written at the same time), then you're looking at maybe 3 albums worth of material since the Black Album about 25 years ago (discounting the covers).
While there's no denying Hetfield probably turned more people onto Mesa than the next 5 guys combined (me included), his relevance over the last decade or two has declined significantly, especially considering he's become known for amps like the VH4 as much in recent times, or used the Axe-FX live on some recent dates. Petrucci has consistently used Mesa and been a champion for them in every album, and Dream Theater consistently churns out new albums and tours to this day (whether you like the albums is subjective).
My long winded way of saying there's probably no one who deserved a signature Mesa over the course of the years more than Hetfield, who has arguably been their highest profile user and used their products on iconic albums with tones people still try to cop today...I just question whether time has closed that window of relevance to where a signature model would even make sense in current day.