For myself I'd see this as one of those flavorful moments where the system can really "Say Yes" to the player and clear the way for some cool and inventive effects to occur, particularly because it's kind of the apogee moment for the Cleric. If the rule was that the cleric ignores the accrued penalties for the day to make the roll, and then gets slammed with the -10 penalty, then it's something I can see really drawing out the flavor of the class.
I concur. That -10 penalty deserves something significant, IMO.
I don't know how Joseph wants to handle this but I think this is one of those things that needs some advice in the Judge's section. Maybe something along the lines of assuming that something
is likely to happen. So rolling a Divine Aid check against the following table.
0-3: Nothing happens. Your deity is distant and has no interest in getting involved. Weep for you are forsaken!
4-6: The deity answers the request. But in the most destructive form imaginable. Take the request, turn it up to 11 and grab a tree trunk. Everything from an earthquake to a mile-wide tornado to burning acid rain to a plague of stinging bugs whose bites fester and rot the skin is possible. The deity is toying with its Cleric, accepting the request as a test of the deity's power but now wanting to test the Cleric's faith in return.
7-10: The deity grudgingly grants the request at one-half efficacy. Yet confronts the character in some manner (perhaps teleporting him to its plane of existence for a brief consultation that last hours in real time or by speaking through the mouth of one of the Cleric's slain victims or by possessing one of the retainers and using the retainer as its mouthpiece, leaving the retainer's lifeless shell behind when it departs). Long story short. The deity is displeased and wants to tell the Cleric about it.
11-14: A simple request is granted no questions asked. A more complicated request is granted at 1/2 efficacy. Moreover, the deity seeks to remind the Cleric of who's the boss. So the Cleric is presented with symbology indicative of the deity for the rest of the day. If the deity has an affinity for thorns, then the Cleric will find a dozen in his boot at the most inopportune time. If the deity likes cats, then cats will jump out at random points during the day to frighten or trip up the Cleric.
15-17: A simple request is granted no questions asked. For more complicated requests, Never send a Wish to do a God's job. The deity possesses the Cleric and then proceeds to wreak holy (or unholy) havoc. The deity is there to grant the Cleric's wish in the most blunt, direct and incontrovertible manner possible. If the Cleric's wish is for his childhood sweetheart to love him again, the deity will find the woman, possess her of a love of the most desperate (and potentially violent kind) and then impregnate the victim with a demi-god -- creating a slave to the Cleric incapable of any emotion but a desperate longing for the Cleric. If the Cleric's wish is to banish a horde of demons, the deity will banish them and then proceed to destroy everything it can see to prevent their return. The deity might also turn on characters with differing alignment. Best tactic in this instance, assume a worshipful, prostrate pose that is non-threatening. Deities do not understand human emotions or even the concept of mortal life. When the deity leaves, the Cleric loses 1d8 points from every attribute besides Luck, only one of which is permanent damage.
18+: Congratulations. Wish granted. This time...