Found a way to do this. Solo the track and export it to an aif or wav file. Import it into your project by dragging it into the tracks window in the space below the existing tracks, making sure you drop it at the start. This imported track now has baked into it all your automation and other processing. You can then set the overall volume for the track using its volume slider while preserving the relative changes established by the automation. Mute or delete the original track. BTW if the original track was mono it will be re-imported as a stereo track, but I see no harm in this. If you had an off-centre pan setting in the original, this will be reflected in the different L and R waveforms.
But if anyone knows a more direct way of doing this within Garageband I'd be glad to hear it.