If you plan to use sorghum syrup (or better yet rice syrup)as a base and are only looking for a kg or 2 of malted millet to add flavor and body to your brew... you can try "psudo malting" some millet or buckwheat. This is my standard practice. Cook grain in a rice cooker, cool to warm room temp, add a solution containing amylase and protease, hold at warm room temp for 3 to 5 hours, then refrigerate overnight. In the morning I add gf rolled oats (dry) and some white sugar...then I cook the whole mess in the oven at 350 untill it is the color of burnt toast (stiring ocasionally) I call this "brewers granola" and I use it in every batch as part of a partial mash. You just need amylase and protease, the rest you can get at the grocery store. I add 1 to 2 kg of this "granola" along with some amylase to a mini mash for each batch.
I have used millet, gf oats, sorghum, and buckwheat, with varying amounts of sugar added. I made two batches today, one millet and oat that is munich-ish, and one millet and buckwheat that is close to crystal malt.