For example, if I asked
What idea does "Fiat justitia ruat caelum" convey?
Is this a question fit for the proposed Law Stack Exchange?
I would say yes.
I can imagine many questions where it would be difficult to draw a line against the philosophy of law. For example, you could ask
There's a factual answer (the right is specifically qualified as to lawful arrest), and a philosophical one (an absolute right to liberty undermines the rule of law). If we said 'no philosophy of law' it would involve lots of arbitrary decisions against perfectly valid questions.