Thanks for that comment and your thoughts! I am unfortunately unfamiliar with the works of Hare, but it sounds interesting and I might have to read up on that.
I totally agree with you, that there are statements to which we assign truth values, that depend on the frame of reference (like “Derek Parfit’s cat is to my left”, or the temporal ordering of spacelike separated events.)
I would also not have a problem with a moral theory, that assigns 2 Utilons to an action in one frame of reference, and 3 Utilons in another.
I do however believe that there are some statements that should not depend on the frame of reference.
We have physical theories to predict the outcome of Measurements, so any sensible physical theory should predict the same outcome to any measurement, whichever frame of reference we use to describe it.
We have moral theories to tell us what actions we should do, so any sensible moral theory should prescribe the same actions, whichever frame of reference we use to describe them.
If you however do not have that requirement to a moral theory, I see that discounting realists would not have to change their views.