it is possible, but doesn't make much sense
A) without the blocking part, this is exactly what StatusSaver does...
B) JnrzLoader doesn't need to block people to sign in offline (NegaPatch neather, but that's a rather dodgy thing to use)
C) if you block everybody on your list, it can take a VERY long time (same with unblocking)
D) What about the people which were already blocked? (true, you could make it that the plugin skip those, but blocking your entire contactlist is rather a dodgy way anyway...)
E) There are much much better ways.... (see my list in my earlier post)
It would be like walking to the busstation to catch a bus and to go to your neighbours house, while you could simply step outside and walk nextdoor... Both ways will get you there, but the first is a bit (well... very) dodgy
/ 
