As this is field-critical and as you note Juju doesn't currently support the ability to bootstrap to multiple subnets I think that the only work around atm is going to be to manually add the machines in the right subnets/zones and then enable-ha --to the machines brought up.
There's work to make zones constraints that might help with this on going but I'm not 100% sure on the tie in between the subnets and the AZ here.
As this is field-critical and as you note Juju doesn't currently support the ability to bootstrap to multiple subnets I think that the only work around atm is going to be to manually add the machines in the right subnets/zones and then enable-ha --to the machines brought up.
There's work to make zones constraints that might help with this on going but I'm not 100% sure on the tie in between the subnets and the AZ here.