`uuid_generate_v1mc` in Postgres “uuid-ossp” module: What is random multicast MAC address?
RFC4122 documentation (section 4.1.6) specifies:


  For UUID version 1, the node field consists of an IEEE 802 MAC
     address, usually the host address.  For systems with multiple IEEE
     802 addresses, any available one can be used.  The lowest addressed
     octet (octet number 10) contains the global/local bit and the
     unicast/multicast bit, and is the first octet of the address
     transmitted on an 802.3 LAN.
  
  For systems with no IEEE address, a randomly or pseudo-randomly
  generated value may be used; see Section 4.5. The multicast bit must
  be set in such addresses, in order that they will never conflict with
  addresses obtained from network cards.


If I interpret this correctly, I'd say: a random Multicast address is any randomly generated MAC adress which has just the multicast bit set. The multicast bit is just one of the bits from the node part of a UUID (for all practical purposes, this just forces one specific bit of the UUID to be set).



Side Notes

I don't think you actually can specify a MAC address to the PostgreSQL function. If it follows the RFC, the function(s) must either use any of the MAC addresses available in your system, or a random one (with a specific bit set). 

Whether this random value is always the same for a specific machine (which wouldn't look very random to me) or is just purely (pseudo)random and changing every time, is not clear from this explanation... but can be very easily tested:

SELECT uuid_generate_v1mc() AS u1, uuid_generate_v1mc() AS u2


gets me right now:

'91ccbe0c-488c-11e7-8d61-b7a8bb0bd0e3','91cd902a-488c-11e7-8d61-8bdf8f55ae02'


This translates (using the PERL program from https://stackoverflow.com/questions/1709600/what-kind-of-data-can-you-extract-from-a-uuid) to

time: Sat Jun  3 20:43:43 2017 +682.51ms
clock id: 36193
Mac: b7:a8:bb:0b:d0:e3
broadcast/multicast bit set.


and

time: Sat Jun  3 20:43:43 2017 +687.889ms
clock id: 36193
Mac: 8b:df:8f:55:ae:02
broadcast/multicast bit set.


... so, the MAC are actually completely (pseudo)random.

As pointed out by @EvanCarrol: I also think you're better off with v4 UUIDs, I don't think you'll get less collision risk with a randomly generated MAC. 

Besides, very many network devices (routers, switches, etc.) have programmable MAC addresses (this is very handy when you want to replace one broken device by another, and make sure all the other devices don't notice any difference at all). This, somehow, makes the MACs not as unique as you probably thought. 

Alternatvies: If you work with Windows, may be this tool can let you fake a MAC address. I've not tried it myself, so, "no strings attached".