As Gompy says, sometimes alt.binz does perform a repair after you manually download additional par files. This may be due to the fact that alt.binz misinterprets the number of blocks that are still available to download. Or, as has been discussed before, due to the so-called Diablo bug, in which case alt.binz believes there are fewer blocks missing than there actually are.
As a somewhat dirty solution to both: If a repair fails, alt.binz could download one or more additional par files and then try to repair again. This could be implemented via an option "If repair fails, download XX additional repair blocks". If repair fails again, the process could be repeated (i.e., download even more blocks). In order not to try too many times, an additional option could set the maximum number of retries (e.g., "Retry failed repairs at most yy times" - a zero would mean never to retry a failed repair).