Mosegaard Hansen, Maj-Britt. The structure of modern standard French : a student grammar. p 86.
As I can see this is a quite similar to this moment in the English:
The imperfect subjunctive is also used in "that clauses" after a wish:
I'd rather that it were more substantial.
I wish she were here.
This last example can be contrasted with I want her to be here, in which the indicative rather than the subjunctive is used because there is a substantial possibility that the hypothesis is (or will be) true.
I do not know any French so I do not know if you actually want to make a point about the subjunctive vs. the conditional mood. If yes, then I am completely off. Many languages, however, only have on of them and use one where other languages use the other.
Je možné, že by Lukáš nemusel být na setkání. (conditional, less probable)
Je možné, že Lukáš nebude na setkání. (future indicative, more probable)
However, I cannot say that only one of them would be strictly possible with "Je možné" (it is possible) or "Je pravděpodobné" (it is probable).