Check out the Low Gain Electronics Short Bus for a good trigger sequence combiner. It's passive, cheap, and fun. You could probably use it to switch between and combine multiple channels of Metron, allowing you to avoid buying Grids. You could have a straight pattern set up on one channel of Metron and a fill pattern on another channel, and then switch back and forth or combine those trigger sequences with the flip of a switch. Switchblade would allow something similar as well.
On a related note, I just checked out a demo for Metropolix. Wow. Awesome.