It seems that the issue is due to the fact the promiscuous mode on the VirtualBox interfaces was set to "Deny" on all interfaces. I did not perform all the tests, but setting promiscuous mode to "Allow All" may save my afternoon.
This post put me on the right track.
I will have to review extensively the wiki page and review the security implications.
Despite the fact the VirtualBox box interface "Intel PRO/1000 MT Desktop" may not allow it, no error shows up in OpenWrt log
device eth2 entered promiscuous mode
Thank you.