Quoting Steve Tremblett (sjt@cisco.com):
> Both ethernet interfaces on my Sparc 5 have the same MAC address after
> boot.  I've been told that I can change the MAC address with "ifconfig
> interface ether address" but it doesn't work at all for me, and isn't
> documented in the manpg for ifconfig.  The source (1.5.3) has no
> reference to "ether" as an address family.

Just to clarify:
Your Sun has a MAC address that all the ethernet devices will use.
Multiple MAC addresses are a side effect of machines that were
made into network workstations after the fact (PCs).  They had
no MAC address available (and no boot prom or other useful things),
so vendors had to include a MAC address with the add on card.

The command you site works fine on SunOS and Solaris.  It was important
when developing HA software as it let us toss a MAC address between
a pair of machines' 'virtual HA interface'.

AFAIK, that command is Sun unique. You haven't mentioned an
actual problem you are trying to solve, so I'll just presume
it's an intellectual exercise and say, "no, you can't easily
do it in NetBSD"  (there was an sea.c program for OpenBSD that
might work - Set Ethernet Address).