Is it safe to plug in your N900 to this - Probably. The micro-USB has a standard pin out for power, ground, and data. Unless the makers of this system are criminally stupid you probably won't hurt your N900. But notice I said probably, the risk is yours.

Will it charge your N900? Maybe. The N900 is more finicky than most devices in what it will accept a charge from. Read this forum for more info about aftermarket chargers.
David