Some grobs can be accessed “laterally” from within another grob’s callback. These are usually listed as “layout objects” in the “Internal properties” section of a grob-interface. The function ly:grob-object is used to access these grobs.
Demonstrated below are some ways of accessing grobs from within a NoteHead callback, but the technique is not limited to NoteHeads. However, the NoteHead callback is particularly important, since it is the implicit callback used by the \tweak command.
The example function defined below ("display-grobs") is probably not that useful, but it demonstrates that the grobs are indeed being accessed.
Example console output:
#<Grob Accidental >
()
#<Grob Stem >