Do you have a name for a "divine warlock"? Might I suggest "cabalist"?
Cabalists could have a huge effect on a campaign world; someone with remove disease as a SLA could single handedly stop a plague. To control this, you might say that a cabalist has to designate who he can heal at the beginning of each day. 3+cha mod living people, or some such. A cabalist couldn't raise a person unless they were part of their "cabal".
Parties with warlocks still have to stop and rest after a few encounters because the cleric runs out of spells. But if there is a cabalist in the party they could potentially go all day. On the other hand, a cleric can pull out all the stops and use all their high level spells in one important encounter; cabalists will not be able to match that intensity, though they can do great on duration.
In general a cabalist should be about as powerful in combat as a cleric who is holding back (i.e. one who is expecting there to be 4 or 5 encounters before a chance to rest). More specifically you have to figure out how well (and at what level) cabalists will handle the following kinds of clerical activities:
- in combat curing
- out of combat curing
- removal of status effects (blindness, deafness, disease, paralysis, curses)
- restoring of negative levels, ability damage and drain
- raising the dead
If a party has a cabalist instead of a cleric, they'll still have to meet the challenges that adventure designers consider appropriate to a particular level. An adventure for 7th level characters will assume that the adventurers have access to cure disease, and can probably neutralize poison as well. So you can't really limit the rate in which cabalists acquire abilities.
Not to mention the fact that you'll have to think about how you'll make it fun for someone to be a cabalist. If all your character can do is cast cure minor wounds it won't be much fun to play, even if you can cure 600 hit points an hour. Some kind of ability to turn or disrupt undead should be an SLA as well. Buffs are a common motif for clerics, and something like heroes feast is essential at high levels.
What you might want to do is to slow down their SLAs a little. Most SLAs can be used only every other round. A mechanic like psionic focus might work; they can use a SLA, but need to make a DC 20 concentration check to regain their focus; until they do so, they can't use their abilities. But other than that, they could be much like a cleric (BAB, armor, hit dice).