The MM-4 really does run DC since it will accept batteries, but it can take the 9vac because it has a bride rectifier that converts it to DC internally. I would think you could since it does take about 16V through the PP2 outlets. I would be more concerned if the brick has enough milliamps to power it though since I'm guessing it takes somewhere in the 200-300 range. Good luck, and if it doesn't work out I'm guessing it has some sort of power protection diode inside.
yeah if it's 1a that's a lot of juice and you should be fine unless you are running a ton of high draw pedals. I have a MM-4 laying around but I don't have any 18v adapters with 2.5mm plugs or I'd give it a try.