Since sets setN, setZ, setQ, setR, setC have their symbols it would be nicht to add other common doublestrike Symbols:
e.g. setH Quaternions setO Octonions setS Sedenions or Surreal Numbers
Well, this could be asked to many project leaders.
The letter-like symbols you mentioned are part of Unicode (math symbol section, letter-like symbols). Then it is the responsibility of Unicode to decide or not to add the symbols.
You could also try to combine basic shapes with Math primitives to get an approximate result. In this case, please share.
Note that Math has not been actively developed for years. It’s quite laging behind the other components’ capabilities.
Last, if your question is in fact a feature request, this is not the place to ask. This site is dedicated to matter-of-fact usage problems and enthusiastic LO aficionados try to find answers and workarounds. For bugs and requests for change, go to the bug site.
If you feel brave and have some time to dedicate, participate in development. Any help is always welcome.