My understanding of it is that EK deploy 3-class a/c on routes where there is a demand for F, and 2-class where there is not (or more demand for J) thus increasing J class seats.
I guess if there was enough interest/queries from travel agents & customers about booking F on that route then they would change the aircraft type. 777 is flown both with and without F.
Interestingly, the demand for seats from Dubai to Jeddah was so great that they added an additonal service- the demand was that it be an A380 and as you can see from their schedule, they put it on. I hear from A380 crew that it is always full to bursting, (Jeddah being around a 2.5hr flight) and yes even in F!
Another nice way to sample first class if you are not sure is to book a leg on one of the shorter sectors, say Bangkok to Hong Kong or Auckland to Sydney.
To know what services your flight has, you can actually search on the EK website by flight number and date to see what the seats, service and IFE are like (obviously subject to last minute operational/aircraft changes) but I find this very handy!! Just click 'Services by Flight'