Very good suggestion – since the light is supposed to be associated with emission then your spectrograph would be an ideal way of tackling what is supposed to be an ephemeral phenomenon that comes and goes. You’ll have to work out a good methodology of offseting to measure the sky and back again – bobbing to and fro on some time-scale to ensure changes in the Earth’s atmosphere are properly subtracted from the Venus signal. Beware of scattered light from the bright side contaminating the signal you are after. You might therefore need to also measure the bright side (with the gain turned down on your spectrograph – or by just measuring the sky north/south of the cusps to quantify this). That makes for 2 or 3 sky positions as well as the dark side of Venus herself.
Richard