Fermented "red" rye malt is an ingredient in some Russian breads, e.g. Borodinsky. It's made commercially by placing green malt in 3-foot piles and letting it get warm and fermented on its own, under temperature controlled conditions. The inside gets hottest and pieces of the inside and outside are exchanged systematically. Some mold is formed in the process. This exact method is close to impossible to reproduce at home and many Russian websites have tried to figure out how to come close.
Actually, the only hard part is the fermentation itself, after the "green" malt has sprouted. One person on a website suggested putting the green malt in a plastic bag that is almost closed, until some mold starts to form. I found a more promising possibility, but I haven't tried it. Actually, I have a supply of around 4 lb. of the commercial fermented malt that I got in Moscow, but I'm just curious about making it at home, in case I may need it in the future.
The promising lead is the malt type called "acidulated malt," which is soured with lactic acid. I'm not sure if acidulated rye malt would be close to fermented rye malt after souring and roasting, but it may come close. The book "The Homebrewer's Garden" has a home recipe for souring green malt on page 136, which I was able to view on acidulated malt. The main thing is if you can get souring without mold.
Here you go http://www.thefreshloaf.com/node/27954/making-red-rye-malt courtesy of our very own Dabrownman.
Red rye malt is difficult to find (well atleast here in London it is) so I bought off the internet the next best thing - crystal rye malt. If you're in the UK i can show you where to purchase some from if you're interested.
These are not the same thing. I actually have a supply of fermented rye malt from Moscow, so I don't need it now. As the Russian sites mention, you can't possibly reproduce the temperatures found in a 3 foot pile of green malt, so the result will differ from commercial fermented malt. Now, on Ebay there are currently two sellers of the real thing. One is from Ukraine and grossly overpriced at $30 for 250 grams; the other is fairly priced, but a huge quantity for $50 from Russia. Maybe someone will eventually sell at a moderate price, such as $10 for 300 or 400 grams. Then I'd be a buyer. I've tried various crystal malts in the US and they are very pleasant, but nowhere near the pungency of the real Russian product.
The idea I brought up in my post was using the lactic acid fermentation of green malt as a substitute for the huge piles that the pros use. This most likely would not be an exact match either, but it may come closer than the other alternatives.