I did something like this. I think it mostly falls under "role-playing options" but it depends on the patron.
First, in setting up the, uh . . . setting . . . I defined the possible patrons a 1st level character could make a pact with, and actually placed them in the megadungeon. The fiendish patron, for example, is sort of the overlord of the introductory area, which is crawling with (weak) demons and devils; and the Great Old One patron is a behemoth sleeping on the floor of a deep underground lake, and as it breathes (at a rate of only twice a day), it creates a tidal effect throughout the megadungeon's waterways which floods areas, reverses the direction of rivers, etc. The fey patron is a Fey Tree that ties the megadungeon to the Feywild, creating something akin to the Genesis Cave from Star Trek 2, and creates a way for me to justify endless spawning fearsome, fey-touched beasts into the megadungeon.
Along with defining the patrons, I added some caveat to the pact to limit the warlock. I didn't use any specific mechanics, and I made no attempt to balance between the pacts . . . and I'm a little terrified that the whole setting would be placed on a ticking timebomb if anyone actually makes a pact with the sleeping behemoth.
So, specifically, this is what I did:
For the fiend, the warlock is prohibited from "attacking the captains of Hell." This essentially o means not attacking any devil who leads a squad. Doing so might break the pact, or maybe the warlock will later be warned (by a sign, or in a dream maybe). If the pact is broken, the warlock will be either prevented from gaining levels in warlock; or perhaps just prevented from learning new invocations as levels are gained. The warlock will be offered a mission to reform the pact. In return, the devils tend to be less aggresive towards the warlock and his allies, and may be more willing to parlay than otherwise.
For the Great Old One, the warlock is not prohibited from anything. Rather, he is tempted by power. Any time he uses a warlock slot to cast a spell, he may choose to draw power from the sleeping behemoth. When he choose to do so, he rolls 1d4, and if he rolls a 1, the spell slot increases by one level, and the warlock may choose to draw more power, this time rolling 1d6, if tgat rolls a 1 the, the slots level is again increased and the warlock can now choose to roll a1d8, and so on althe way up to a d20 or 9th level. While in an area influenced by the behemoth (mostly, the tidal waterways), the warlock does not get this choice. His patron's power is insidious, and the warlock must roll the dice evvery time he uses a warlocm slot. The drawback to this is, well, a ticking timebomb: If the warlock ever manages to cast a 9th level warlock slot, the Great Old One wakes from his slumber.
The fey patron just requires the warlock to pick its fruit and plant its seed in the center of the city above the dungeon. The tree is giving the warlock power for this purpose only (reaching the tree is no easy task), and doesn't care what the warlock does with the power so long as the warlock eventually fulfills his end of the pact. So, this is the "no restriction or consequences," pact, so it gets no extra benefits either.