A context-independent rational morality might be doomed, but we actually do live in a specific context, and you can be arbitrarily rational with respect to that context.
And regardless, you have to value those things which promote decision-making capability, since you'll end up a non-moral corpse without it, and sacrifice whatever local good you're trying to save in the process. So there are very large contexts with clear moral foundations that you are a part of.
That being said, since you have to define what "good" means, and what the role/fundamental characteristic of various contexts is, I think any attempt at a completely rational morality is quite doomed.