Great question about Unicode music symbol support! I've run into similar challenges when working with music notation symbols across different platforms.
For anyone working with Unicode music symbols, I've found that having a reliable reference can be incredibly helpful. I actually created a resource at https://musicsymbols.org/ that lets you easily copy Unicode music symbols - it includes sharps, flats, double sharps, double flats, and many other notation symbols. Each symbol shows its Unicode value and can be copied with a single click.
Regarding Pd compatibility, in my experience the basic sharp (♯) and flat (♭) symbols you mentioned generally work well on most systems, but the more specialized symbols like double sharps (𝄪) and double flats (𝄫) can be hit-or-miss depending on the font support. You might want to implement a fallback system where you detect if the Unicode symbol renders properly, and if not, fall back to your "lame" ASCII alternatives.
Have you considered testing your abstraction on different OS/font combinations to see which symbols render consistently?