I know you can use unarmed strikes to deliver spells that need a touch attack.
However, is it also possible use unarmed strikes with spells that allow for several touch attacks, like Darkfire (SP, p59)?
you can strike opponents with a melee touch attack, dealing 1d6 points of damage per 2 caster levels
So could a 11th level Monk with 5 levels of Cleric cast this spell and deliver 4 times this damage using Flurry of Blows?
Yes, you can
On Holding the Charge, it says:
This means that instead of trying to touch your target with a standard action, you could use a full-round action (such as Flurry of Blows or a Full Attack) and attempt to deliver a held charge on each of your unarmed attacks. Keeping in mind that each attack will provoke attacks of opportunity unless you have the proper abilities to prevent those (like Improved Unarmed Strike or similar).
The wording on Darkfire is very similar to Chill Touch, or even Produce Flame, which are common spells to use like this.