Was done decades ago
Surely they have just re-invented the wheel?
In the 1980's I had a Spectrum and bought a paperback entitled "Over the spectrum". This contained a complete disassembly of the ROM code together with the entry and exit parameters of all the many subroutines in the ROM.
Nowadays there's a well-commented online disassembly of the Spectrum ROM here
http://primrosebank.net/computers/zxspectrum/docs/CompleteSpectrumROMDisassemblyThe.pdf
I wrote some assembler code on the Spectrum to provide a perpetual almanac and sight reduction calculations, which was used for astro-navigation on a small yacht for a while. Eventually replaced by an HP-41C programable calculator which did the same thing without the need for a power-gobbling portable TV while also saving all parameters to use for the next day's ded. reckoning, set & drift and course to steer calculations, which the Spectrum did not do (though I could have programmed it to save to tape I suppose).
More recently I translated the Z80 code into ARM assember, doing so on an almost a line-by-line basis (but re-writing the I/O routines to be compatible with my ARM hardware board). It worked great - though obviously way faster than the Spectrum so software timing loops had to be tweaked. As it is not a Z80 emulator it will not run any Spectrum binary code programs, but works OK with spectrum Basic programs.