What a way to go. A Martini with a splash of the green fairy. I like to use old tom gin to make an Obituary as it sits so much better with the absinthe...
You could. The flavour profile will be a bit different as they both do not have the same herbal qualities. The Chartreuse may make the cocktail a bit sweeter. While you would have the green colour, you will be missing the unique coluration that the absinthe give the final cocktail.