Most hacking scenes (be it games or consoles as a whole) are proportional to their popularity, and ease of getting ROMs (16 meg SNES or GBA game is nothing and possibly HTTP if needed, several gigs of DC iso in an unknown format and likely not on HTTP*. Dreamcast has a bit of benefit from its quite notable homebrew scene but it is still not one of the big boys.
*granted I have not looked for a while but even when the DS was beating out DC sizes it still did not see that much there.
At this point anything PS360 and older is blown fully wide open (hardware options vary a bit in difficulty, and how well the laser functions in the case of the old stuff as replacements are getting tricky and ODEs are similarly nowhere to be seen) and can do anything that is possible in hardware. As mentioned above then most of it is going to involve playing with disassembled code then time investments creep up over things you might have code for, give or take the small window and presently smaller library of decompiled games for the PS1 through early PS2 maybe and what little can be teased from Android Java remakes or general source releases.
The quality of the debugging emulators varies dramatically, and again is usually proportional to console popularity. Nothing is close to what you can do with PC debugging and the Dreamcast will be considerably further down the list there.
You should be able to find something with a decent cheat engine, breakpoints and and assembler though which is fair enough and suitable for most hacking projects on most systems. Dreamcast CPU and hardware is not as crazy as the Saturn's but MIPS is not so bad and its 3d kind of makes sense in the end.
Depending upon the setup you might do better finding an older emulator, debug fork, hoping it had GDB bridge options or something similar.
Said emulators are also where most will find the hardware info for their devices.
You will be lucky to find nice common decompression tools (not sure what it provided at BIOS or SDK level and things I have seen are still pretty custom), plugins for your chosen 3d modeller to import/export trivially or niceties you might see elsewhere. It is also old enough that you will not encounter so many as seen on PC/whatever tool formats and many things will be rather more custom to/mirroring the hardware quirks.
In the end it being part of the "everything is fully owned" set means you can pull ISOs apart (few different formats out there but tools to do it have existed since it was current), look at the files it contains, analyse them from a general programming perspective, from other games/instances of the format, play with an emulator (some quasi hardware debugging exists but don't go there) and debugger, do a what does this button do approach (not like you have to burn a disc to check what happens when) and whatever else. This is the state of being for most ROM hackers, though you might be further in the dark as there might not be so many knowing said flavour of MIPS and associated hardware that you might be able to bounce ideas and speak to when you hit a wall compared to NES, SNES, megadrive/genesis (though even that is tricky at times), GB/GBC, GBA and DS.
