I think Cosmed and others have a portable VO2 tester available for a few years now. Still very expensive for individual use and not necessary for controlling training. As you note the metabolic response to exercise can vary significantly from day to day. This will be reflected in HR and in O2 uptake. Got tired legs one day? HR will be low and perceived effort high. On those days you’d see lower O2 uptake numbers. The O2 uptake is a product of how much power the muscles can produce and if the muscles are tired there won’t be much demand on the heart (low HR) and likewise the O2 uptake will be lower than on a day when you are feeling fresh. There is a correlation between HR and VO2. It is linear in the aerobic zones (1, 2 and into 3). It becomes non-linear around the anaerobic threshold and increasingly so in Z4-5 where HR doesn’t do a good job of reflecting effort. At these intensities the anaerobic system is contributing considerable energy to the mix and it does not O2 so is independent of HR.
But this is getting way, way down in the weeds for the control of daily training. Since the relationship of your metabolism to HR or VO2 changes depending on recovery state the test you took last week or last month only gave a snap shot on that day. So regardless how you tested, in a lab running or outside skiing with a Cosmed you have only rough guidelines to control intensity TODAY. For our sports HR works reasonably well for everyone from duffers to World Champions. You just can’t go parsing the metabolism to finely. Thats why these 7 zones systems are false precision. Really 3 zones works quite well.