I have done it with Broccoli, here is a picture.
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Here is a planting group
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I started them in 1801 trays, each cell is about 3.5 inches square. I put the seeds towards the four corners of the pots.

It worked well, actually it was my last good crop of broccoli. Since then it has been ravaged by drought and heat.

Jay

I think it would depend on the crop. Things that are planted multiple seeds per hill should be ok, but I wouldn't tried broccoli, since the heads are usually determined by how much room they have to grow. I've done it with basil without any problem, since I transplanted again later into a larger pot.

Marla

Neat,Jay, Thanks!

yep, basil is easy to start heavy. I just transplanted out 72 plugs with and average of 5 very strong plants each. Still, not sure if this is crowding it in the garden? Any possibility they would be better planted as individual plants instead of in bunches? Basil has done very well, but I loose control of it after two or three cuttings and have to just mow it down and let it come back with nice stuff.

I am looking forward to trying beets in blocks on 6"x16" spacing.