Here’s how you can make Viscosity set the MAC address of your TUN/TAP device prior to establishing a connection. Under the Advanced tab of your connection definition, simply add the following under “Extra OpenVPN configuration commands …”
Viscosity uses tuntaposx internally and I found this thread on the tuntaposx-users mailing list where the devs explain why they chose not to use the 00:FF paradigm like under Windows. IMHO, it’s very helpful for administrators to be able to distinguish between local MAC addresses and VPN MAC addresses (i.e. like in the dhcpd.conf file).
I hope this will save some time for future users of Viscosity who have the same problem.