I received a reply from Becker Support in October 2016 to effect that an "internal change request" is being implemented to "edit" the firmware so SIL and SDA settings can be altered. They promised to keep me updated in the matter, but I'm guessing it's going to take a while for the work to be completed. No telling what they would want to charge for the eventual firmware update?
At Flyer Live, I learned of the new Trig TN72 TABS certified GPS source, which has a SIL = 1. The source is reasonably priced at £350 (+VAT??), and I emailed Harry Mendelssohn, as Becker agent, to find out whether this could legally be connected to the BXP6401 to give ADS-B Out. Presumably with this SIL status, you would be visible to TCAS etc. so advantageous over a non-certified source with SIL of 0 (which will be ignored by TCAS). The HM technical guys are meant to be following this up with Becker, with whom they are regularly in contact, because I am unclear whether the SIL of the GPS source would be transmitted in the ADS-B transmission, or would it be the SIL (=0) of the Transponder which is transmitted, thus negating the advantage of having a certified source? Yet to receive any response to these latter questions.