I don't know of any way to change the SBRS value locally. But even if that were possible, it isn't possible to do so with SPF or DKIM. The SPF verdict can be queried in message or content filters, but that's all. And you can't evaluate the DKIM status in the HAT because the message isn't received until after the HAT processing has already been done.
We handle the problem of "legitimate sender with a bad SBRS" by having a separate sender group that disables reputation filtering but not IronPort Anti-Spam. When a legitimate sender contacts us to complain about this problem, we put them into this sender group for a while until their reputation recovers (assuming that they aren't still spamming). This sender group has to be put into the HAT above the sender groups which are based on SBRS, so it will override them.