First of all, thx for the firmware link. Next monday i'll try it on 3 of the 6 printers and, if it works, on all of them.
Meanwhile, i've got a solution that, at least, keeps the user satisfied: set a ping to the printer with the -t modifier, so it's receiving a "wake up" every second. Not a final solution but it works!
Ping xxx.xxx.xxx.xxx -t