There's been a handful of threads discussing the lack of PPPoA support in RouterOS - mostly from the American side of the pond, DSL users, grumbling about the lack of PPPoA support.
I can't say this will work with all DSL providers, since I only have one to test with, but I was able to get my Mikrotik RouterOS install to authenticate against Qwest, who most definitely uses PPPoA.
I have an Actiontec router, which... doesn't handle load well. Under heavy network traffic, I reboot it weekly to keep performance tolerable.
Doing some reading, it seems PPPoE and PPPoA are very similar - just over different mediums (ethernet vs ATM).
On a whim, I tried setting my DSL modem into "RFC 1483 Bridging Mode" - it basically makes the modem an ethernet to DSL adapter.
I set up a PPPoE client in my router, on the interface facing the DSL modem. I used the same PPPoA username/password the DSL modem had, and... IT WORKED!
I'm now running my network through my Mikrotik router, PPPoE authenticated to my DSL network via the DSL modem in bridging mode.
And, as a bonus, it seems to be bridging ethernet back to wireless. So I can separate the untrusted wireless side of my network from the secure wired side.
I don't know if this works across all DSL providers, but it certainly works on mine (posting through the link now). If a few other people could try this, it might be a solution for those of us looking to put a public IP on our Mikrotiks through a DSL connection.