I'm wondering about heart rate data during a flat(-ish) Individual time trial that takes a bit below 1 hour to complete at elite level, like the typical ITTs in TdF. I know HR alone is far from the big picture but I remember reading about average HR during marathons and variability was remarkably low, irrespective of finish time. So does anyone know the average/max heart rates of well trained cyclists during such efforts?
This is absolutely specific on the heart rate zones of each individual. Each person has a different heart rate at their functional threshold (FTHR). Professional cyclists are able to cycle for a long time (most of the road race) almost at their functional threshold but the actual heart rate is completely individual. Even the percentage of the functional threshold heart rate to some "maximum" heart rate will be individual. All these values must be determined by an appropriate physiological stress test to be reasonably accurate.
See, for example, https://cyklopedia.cc/cycling-tips/what-is-ftp-in-cycling/#1
When it comes to the maximum heart rate, that is not a very reliable value (may vary strongly for different activities) and it is much better to start all you zones and comparisons from your FTHR than some percentage of some maximum.
As a good source of example pro-cyclist heart rates you can use https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6572307/ although it uses a longer race.
The HR during the race was 158.9 ± 10.6 bpm (86.4 ± 2.2% HRmax), and exercise intensity is categorized as vigorous intensity.
As you can see, the variance for the heart rate is higher (more individual) than the variance of the maximum percentage.