It might be possible to create a mod to do this relatively reliably and without killing the server, but it won't become a standard feature of SMF any time soon.
Such a mod could use an approach similar to the one I used in 2.1's set_tld_regex() function, by periodically downloading and processing
this official file to build an array of substitutions to normalize confusable characters and strings. This array would need to be stored in the database somewhere, and one would want to add a column to the members table that recorded each member's normalized username to use for comparisons.