Out of context the first phrase could mean either that you still work there or that you no longer work there. The second phrase can mean only that you are still working there.
Edit
@Chaoasamoeba's answer provides a context in which having been working there means the speaker was working there at the time of the statement but is not necessarily still working there now.
Has yet to [infinitive] is perfectly correct, and there is no conflicting tenses in it. Remember, the verb have can combine with a past participle to form perfect constructions (present perfect and past perfect, respectively, for present and past have), but it can also combine with a to-infinitive.
Usually, have to X means ‘must X’, but if you add yet, the meaning changes. It is in fact a slightly different collocation of verbs, and it works with both be and have, though have is much more common. The meaning of X is/has yet to happen is “X has not happened yet”.
This is why it does not fit your sentence here: “No chess-playing computer has yet to win a championship” means “No chess-playing computer has not yet won a championship”… and that’s frightfully difficult to parse. Presumably it would mean that every chess-playing computer has won a championship already, but it’s basically nonsensical.
The following two are more or less equivalent, though:
…even though no chess-playing computer has yet won a championship.
…even though chess-playing computers have/are yet to win a(ny) championship(s).
The way this idiomatic phrasing works is probably easiest to understand if you use be as the basis: “He is to do it” means either a) ‘he must do it’ or b) ‘it is expected/known that he will do it’. Consider “He is about to do it”—that means that he will do it in just a moment. Yet here means ‘still’, so if you say “He is yet/still to do it”, it means that it is still expected/known that he will do it; in other words, he hasn’t done it yet.
Best Answer
Going by your question, I think it's possible you've misinterpreted what Eliot was trying to say here. Your alternative would result in "Blessed is the man who has nothing to say" (let's ignore the other part of the sentence for now) which would imply that simply not having anything to say makes man blessed, which is not the point.
"Having nothing to say" is an example of a dependent clause, meaning your second sentence wouldn't work as you need a clause following it.
What he's saying is that he considers a man who doesn't talk a lot when he has nothing to say is blessed. What you could do is say "Blessed is the man who has nothing to say and abstains from giving evidence with words." or something similar.
Going to your question sentence, it would have to be
where X is something painters did because they learned the grass was green.