I've been doing some test, but seems BlackShark make a lot more research! I was about to post about the flag location and found he already did quite the research.
It's curious that they allow for the 8 legendaries in the structure, while the game only allows to transfer 6 pokémon at a time (legendaries + catched) for some reason.
I made a quick test and I managed to re-receive some of the legendaries, but not all of them. What I did was get my savegame that hadn't made any Dream Radar connection, get the 0x25E00 value and paste it in a save that has received 7/8 legendaries at 0x25E00 and 0x7F004, I also cleared 0x25E04 and 0x25E08.
I'm gonna set 0x25E00 to 0x00 and see what happens when transfering something over.
EDIT: I just noticed that after receiving landorus on the 0x00 file, the dream radar block still has the 0x80808080 for tornadus, so maybe what's needed is to clear the flags and those "identificators".
The value I got at 0x25E00 after a single transfer when it was all zeroed is "86 FF A0 F1" (direct hex view).
EDIT 2: I've just realized that for our purposes, we don't really need to know how it exactly works, we can just reset the whole thing to an "unused" state, as if dream radar was never used in that savegame, there's really no need to individually clear each flag.