I would use the technique in Create Text Expansion From Selection Macro. Just use slightly different XML for the macro, instead of having the Typed String trigger, just put the macro in your palette macro group and it will immediately appear in the palette.
So in summary:
- Macro Group configured as displaying a macro palette (in Safari or whatever).
- Variant of the Create Text Expansion From Selection Macro action that creates a macro in the above macro group, no trigger required. This macro could be triggered any way you like, and could even live in the macro palette group as well.