I've used Grimaldi, but only returning Bs As to Tilbury. As the bike wasn't on the ship's manifest as cargo, I just rode out of the port. However, in Bs As you will have to sort out the temporary import paperwork and get insurance. I'm planning to return by Grimaldi this November so will be very interested in your experience if you use them. Anything I learn in the meantime I'll post to this thread.
In theory it should be perfectly straightforward, the same procedure as crossing from Chile to Argentina for example, which I did many times. However, as Mollydog says, there may be some palms to grease.
Give me a ring on 01452 424545 anytime if you want a chat about it.