As I understand it, an "spc" file is just a dump of the RAM of the SPC700. (That's why every SPC file is exactly the same size.) So, there's no way to "seek out" an .spc file, since the only thing you'll find in a SNES ROM is code to write to the SPC700 RAM in various ways.
There's something called SNSF, which, like NSF, is basically constructed by carefully, manually stripping a ROM down to nothing but its music-playing code. For certain games (Wario's Woods, for instance), .spc dumping is impossible and that is the only way to play the music.