No, because of the TRX signature used to validate the firmware. These never updated their TRX headers to remain compatible. I can't speak for DD-WRT, but in Tomato's case I even put two Tomato devs in touch with Asus (at Asus's request) so they could work it out together.
Bootloader has nothing to do with it.
Tomato doesn't support the RT-AC86U because it's a major architecture change, and right now there's no know-how or interested in the Tomato dev community toward supporting it - something that would require a LOT of work. Tomato was simply never designed to support anything but the old Broadcom SDKs.
Nothing is preventing DD-WRT from supporting the RT-AC86U. It's up to them ultimately to decide if they want to support it or not (based on time, availability of a development sample, interest, etc...)
Kong might have contacts with Netgear helping him get development samples, considering he's the one who primarily implemented/finetuned support for the R7000 and R7800, which could explain why the R8000P is getting supported.