I need to use a multihomed RouterOS device to take two upstream untagged ports and join them together into a single downstream tagged port. Normally this would be a trivial application of bridge vlans, but in this application the upstream network does authentication based on MAC address. Since bridges only have a single admin-mac, that means the bridge would present the same MAC address on both upstream ports.

How do I configure a Mikrotik device to be multihomed on two different ports, presenting a different MAC address to each? I do not need any form of STP to be working.

Here's one option I came up with, can anyone advise on a less crazy way to do it?

Create a bridge with VLANs as per normal, set a ethernet switch rule to block packets from the admin MAC, create a VLAN interface on the bridge for each VLAN, create a single-member bridge on top of the VLAN interface with a manual different admin-mac.

I would use /interface bridge filter and /interface bridge nat rules instead, as described here. Don't miss the last post!

Interesting. So you'd use those to rewrite the admin-mac on the second port? Do the bridge nat rules work on traffic that comes from the router itself?

Do the bridge nat rules work on traffic that comes from the router itself?

Yes.