Don't think it'll fly
My understanding of erasure codes is that, given any 20 of those 28 chunks that 20 chunks-worth of data is spread across, you can reconstruct the original 20 chunks. Conversely, if you look at any one of the 28 chunks, you cannot tell anything about any part of the original 20 chunks — you need another 19 before you can do that. This means that, by the time a chunk has hit a (somewhat) intelligent drive, it can't usefully be indexed or searched on the drive itself. So indexing has to be done on the original 20 chunks before they hit the array, resulting in more chunks which, with added redundancy, also hit the array.
"Or am I talking bo**ocks?" (as another poster put it).