You can toast easily enough to get the darker grains from your sorghum and your oats. But making crystal is a bit of PITA and to be honest, you can get the same sort of flavours from using sugars. Caramels, candi sugars, demerra sugar, raw sugar, golden syrup, maple sugar, palm sugar, treacle, molasses, honey... so many different types and they should see you right for your toffee, caramel type flavours. Save you a bit of trouble.
If you're searching for substitutes for grains you (as a gluten tolerant brewer) know about - then I suggest a quick way to match flavour profiles with something you think might be a good GF replacement ingredient, is to make a tea. Steep the grain you would normally use in a cup of hot water - cool it down. Now steep the candidates for becoming a replacement... then try to match the flavours between the original and the new... don't be afraid to blend the new stuff a little. You are your friend's guide taste-buds in this, he cant taste the original... but you can.
This will take a bit of effort... beer is made from barley malt and to try and get something that tastes the same, but not use barley - its pretty damn hard. I strongly suggest a series of small batches. Just 1G in a growler or something like that.
As an AG brewer, you're going to have to suspend any snobbishness you might feel about adjuncts, if you want to pull off the GF beer thing, you need all the resources at your disposal and most of the adjuncts are gluten free.
Oh - and have a read of these documents
http://docs.google.com/fileview?id=0B75 ... MTYy&hl=enhttp://docs.google.com/fileview?id=0B75 ... ZTU4&hl=enThey were written by Andrew Lavery when he was a homebrewer - now he owns and runs
O'brien Brewing a Gluten Free micro brewery here in Australia.
Cheers
Thirsty