For the record the megadrive/genesis also has a Z80 used as a sound core (though it can reach out and touch the rest of the system if it has to). On top of that as with all pre/non filesystem using setups you can find actual useful parts of the binary here, there and everywhere or indeed I suppose it is technically possible to find 68K code in the RAM depending upon the game (speed wise, cart being memory mapped, lack of writable memory wise and a general lack of bankswitching means I would not bet heavily on each and every game doing it but it is possible and not all that bad an idea, it is how the Z80 works as well).
Here is where I would probably share a trick like the one for the GBA to find its binary but I am afraid I do not really have one for the megadrive/genesis. Some games will have checksums for various things, you try watching one of these but just as often it could be for everything or no machine code at all.
That said the usual tricks of "watch the bootup and after the I/O is done you probably have your interesting stuff" and "wait until it is in the game and you probably have a general idea of where things are" (again it can and is not unreasonable to skip around everywhere) work well enough. There may well be a trick but I do not know it; I am not the best megadrive/genesis by any means and when I do play with it I tend to be more focused on a given thing (single functions and tracing guided) and not just general disassembly.
Likewise the Z80 code is typically copied to the Z80 by the 68000 (take your pick on methods) via a known sequence (BUS REQ ON followed by BUS RESET OFF is how the main manual deals with it) so you could trace that way for the Z80.