I get that this can be convoluted to make the zone designations completely general, and I get that Z0 is reserved for “no zone”.
Could a work-around be to keep the existing zones as they are but name the recovery zone something else beside “Z0”, for instance “ZR” (R for recovery).
This is a fairly important topic if you are well conditioned enough that you need to do “zone 1” work instead of “zone 2” for your easy runs. You want to count your hours in zone 1 per week, but something like a stroll or some yoga should not really be counted with your easy runs.
In the “uphill athlete” for instance zone 2 is 90% to 100% of AeT, while zone 1 is 80% to 90%.