My understanding is that the subjunctive imperfect tense is rarely used in spoken language.
But then, what would you use in a case where you would need it?
For example, to translate the rather forced example "He was unhappy although he was working," wouldn't working turn out to be subjunctive imperfect? Il était malheureux bien qu'il travaillât ?
How should this work?
The subjunctive imperfect is not always used in formal writing even when it should be, and basically never used in speech. Using it in speech is not only formal but definitely stilted. It is surely de rigueur for a speech at the Académie française but nobody uses it in ordinary life.
Where formal grammar requires a subjunctive imperfect, normal speech substitutes either a subjunctive present or an indicative imperfect. The subjunctive present is the most common substitution, but here it doesn't work, I'm not sure why. Medium-formality speech and informal writing might use the indicative imperfect. Using bien que with an indicative present is rather shocking to hear, but with the imperfect, it can be acceptable.
Il était malheureux bien qu'il travaillait.
But it would be more common to avoid having to use the subjunctive altogether.
Il était malheureux, et pourtant il travaillait.
You are correct, subjunctive imperfect is no different than subjunctive present, except for the explicit past tense. In spoken language it is common practice to substitute one for the other (or often indicative imperfect, which is broadly used) though, as Random suggested above.
I always assumed it is often disregarded in modern spoken language because of its akwardness - learning that tense tends to give nightmares even to most french-speaking kids :)
As a non-native speaker, I really try to avoid the subjunctive whenever possible.
Of course, just not using it when perhaps it should be used is certainly an option (and it's one that I employ quite/too often), but whenever I’m aware of a legitimate, nearly synonymous, indicative alternative to the “ … que + subjonctif” clause in question, I try to remember to use it.
In the case of “bien que,” my go-to subjunctive-avoiding alternative to try to capture “although/even though” is:
“même si.”
So I’d translate your sentence as:
“Il était malheureux, même s'il travaillait/même s'il avait un boulot."
Although this might be an incorrect/less precise/less fancy translation, I personally find it a bit less harsh on my inner ear than hearing myself using the indicative with a well-known “ … que + subjonctif" clause.
