No no, the desired option is to send left-alt to the guest, not to read it from the host.
However it's not correct that macOS doesn't distinguish between left- and right- modifier keys. Here is an application that demonstrates this using entirely userspace calls: https://apps.apple.com/us/app/key-codes/id414568915?mt=12 - click the (I) icon next to a modifier event to see the corresponding enumerations from https://opensource.apple.com/source...DSystem/IOKit/hidsystem/IOLLEvent.h.auto.html
However it's not correct that macOS doesn't distinguish between left- and right- modifier keys. Here is an application that demonstrates this using entirely userspace calls: https://apps.apple.com/us/app/key-codes/id414568915?mt=12 - click the (I) icon next to a modifier event to see the corresponding enumerations from https://opensource.apple.com/source...DSystem/IOKit/hidsystem/IOLLEvent.h.auto.html
