You need a KLIA Ekpress ticket before you are allowed though to the airlines check-in counters (which should not be a problem since you can pre-purchase the tickets in the morning)
Technically you can only check-in 4 hrs before your flight, but I am not sure whether it still holds since you can do online check-in 48hr before flight (i.e if it works, online check-in hasnt been working the past 24hrs
)
I cant be absolutely sure (since I dont use them) but I thought I have seen the plastic baggage wrapping service at Sentral.