I would think getting TPs and avios.com (ex-Airmiles) credit for the same sector would not work as you'd need the BAEC number in the system for the TPs to be awarded. Ready to be corrected, though.
If you don't need the TPs urgently, it may be worthwhile to go for the avios.com credit and then call BA for the seat request. Or phone them first for advice, though I suspect the enquiry might be a little esoteric.