I couldn't find any evidence that anyone had tried this, so here you go:
In a new and unmodded world (version 40d) I embarked with 7 peasants to a random hillside location and assigned three of them to dig different tunnels (to my knowledge rock type doesn't affect the experience gain from mining, but I figured it was an easy way to be sure).
All three gained Novice Miner after digging out 50 tiles of rock each.
I reverted to a backup copy of the world, and added the [SLOW_LEARNER] tag to the dwarven entry in creature_standard.txt (I don't know if it matters, but I added it directly below the [INTELLIGENT] tag).
I embarked to the same location with 7 new peasants, and assigned them to dig straight tunnels in the same places as before. This time they couldn't finish before taking a booze break; I made sure they returned to the same tunnels they started.
Each gained Novice Miner after 84 tiles of digging.
50*1.67 = 83.50
I didn't have time to test this further, so I suppose it's possible that there is a sliding scale or something. I hope so, as a 67% increase in experience required to gain a skill level will still create invincible champions far too quickly. Still, assuming all jobs are equally affected, then those of us who feel that it is too easy to create an ultra-powerful military will be able to lessen the problem somewhat.
I don't have time to play much right now, so I'd like to hear from anyone who tries this out whether strange moods will no longer give enough experience to create legendary dwarves.
Also, I'd love it if someone managed to hack this to create a *3 or *4 or *5 multiplier, but I'm afraid that goes far beyond my skills.