Hypothetically let's say a person called Jack has a game that they have purchased (Lets say an Acorn Electron Game - Twin Kingdom Valley) and Jack cannot play it anymore because his Electron died. Jack cannot buy an Electron anymore (hypothetically) but Jack is a budding coder and wants to port it to the PC for his own use. Unfortunately the game cannot be accessed via an emulator because it has copy protection (it did) so Jack bypass(es) the protection to get at the source code - safe in the knowledge that where fair use comes into play, Jack is covered.
Bypassing copy protection does not give you the 'source code' The source code is what gets compiled or assembled to create the distributed binary.
The DMCA has various problems, and until it's tested in specific cases it's impossible to tell you exactly what the ruling would be. What actually *counts* as protection? The game 'Pairs Love' which is supported in MAME has protection. When it was first emulated it just worked, the MAME cpu core didn't emulate the exceptions which the protection routine triggered when the protection check failed. The 'protection' was so weak and badly implemented it may as well have not existed, and nobody even noticed it until some bug fixes were made to the CPU core and it actually started triggering the exceptions, prior to that it worked fine! Even after that, the only 'protection' was returning a magic number from a certain read. It's actually hard to know if it really is protection, because you could defeat it without trying by putting a random number generator in and resetting a few times (which is a pretty normal thing to do during development for unknown inputs anyway) Likewise, sometimes you'll get sound / graphic roms where the data is in an odd order, but it's easy to see what the order should be. You could call that some kind of encryption / scrambling if you wanted, but on the other hand it could just simply be due to the way the pcb was designed, shorter traces to save money / simplify the design.
Also take the old game 'Civilization' it has a copy protection scheme which involved you having to tell the game which technology came before/after another one. I 'circumvented' that by remembering all of them, and I'd also made a chart of the technology advances for my own gameplaying reference anyway. Would me remembering all the combinations be illegal under the DMCA? Would my chart have been illegal? Would guides on GameFAQs explaining the technology chain be illegal? Most of it was just common sense, and knowledge, is that illegal? It was just facts about how the game worked which also happened to be used as protection. MAME contains facts about how the hardware works derived from reverse engineering, and isn't much different to this example in reality, it's stuff that's been figured out and learnt as part of, in the case of the game, playing, in the case of MAME, researching.
It's not the only game.. With my original Amiga copy of Pinball Fantasies, there was a word based protection system.. about 1/5th of the time the word was something simple, like 'ball', so on the disc I wrote 'just keep typing ball until it works' because it was far easier than trying to count lines and words in the manual, I guess I 'circumvented' that protection system too just through normal use of the game, and solving a problem (why should I look through the manual when I know that more often than not the word is 'ball'?). Do you honestly think anybody could possibly rule against that if it happened today?
You can attempt to apply the DMCA to anything and everything if you really want, and you can come up with some pretty comical examples. I consider your emulation example to be just as ludicrous as the above ones.
The DMCA has clauses for interoperatibility, which is exactly what emulation is, it allows the original software and data to interact with different software / data, but again it's not entirely clear. Your character Jack could quite easily claim he was doing what he did in the name of interoperatibility. There are reasons a lot of people claim it's a 'broken' law, and reasons why it so often gets abused. Again, the only real way you're going to find out is to test it, and I think companies are only likely to want to test it in cases where they think they'll get a favorable outcome and there is significant money involved as to avoid setting precedents which could blow a hole in the entire thing.