I think this is an interesting proposal, but it would need to be extremely clear in the rule what is allowed and what isn't. The OP already does that by specifying contiguous albums and multi-act songs, but we can go further. I doubt this rule will be used very often, so it's worth being incredibly... ostentatious(?) about its implementation.
To indefinitely prevent abuse of this rule, I would add an exact length requirement, since many people might try to map EPs or LPs with very few songs in this manner, such as
this 8 minute map of a mini-album. Obviously, this is not something that you would want split into 5 separate difficulties, all under 2 minutes each. I think that this length requirement logically carries over to multi-act songs for the same reason, and this would be fine since the proposal is obviously meant to be applied to these extreme cases of marathons (over 30 minutes) and not merely 10-15 minutes long maps. From what I know about music and the world, a full-length album is generally over 30 minutes, so that's where I would place the cutoff.
I don't think an example map is necessary if the rule is as clear-cut as I'm suggesting. Once somebody makes a modern example, people will look at that, but it's unwise to include maps in the ranking criteria without knowing how the game will look in the future.
Also, something may need to be added for storyboards (if some man ever decides to do that), cause in my knowledge, maps don't end until the storyboard ends, so if someone maps the first 5 minutes of a 30-minute file with a storyboard, the player will be sitting there for 25 more minutes waiting for the results screen. The mapper would have to cut down the audio file and link it to the separate difficulties to avoid this. This might be able to be edited in the .osu file to end the map earlier than the storyboard ends, but I don't know enough about the subject to confirm or deny that, so I'll leave it for discussion.
With that, I propose a leaner version of the rule:
Single-difficulty marathon beatmaps of albums or multi-act songs over 30 minutes in drain time may include individual difficulties for each song or act. These extra difficulties are exempt from other rules and guidelines regarding duplicate content, audio file coverage, and drain time spread requirements.