Letter of explanation about what, exactly? That the rules shouldn´t apply to you, for some reason?
I realize you are in a tough situation. If you manage to get an ITA and are able to submit a COMPLETE application before 11 December (you need to give yourself an extra day to receive the AOR and then apply for the BOWP), and are willing to sacrifice the application fee, you could do it and apply for the BOWP. But if your application is rejected incomplete, your BOWP will be refused and you will be out of status retroactively to the date your work permit expires.
This could work (not saying it will work), only because eligibility is assessed after passing the completeness check, and you only need to pass completeness to be eligible for the BOWP. But again, it would be costly and you will have to be prepared to have your application refused, and re-apply. That is the best case scenario (unless a VO makes an error in your favor - stranger things have happened).
It would still be possible for your PR application to be refused as ineligible before the BOWP is issued, in which case you would really be out of luck.