I don't have my books with me, but I recall that they focus on the combat related aspects of summoning and not the 'out of the box' thinking.
Which I think is kinda sad.
Summons are like any other power, limited to the 'end of the encounter' rule of 5 minutes outside of combat.
While I wouldn't allow the succubi to make the actual diplomancy check for you, summoning her to act as your 'voice' could net you some strong modifiers to the skill check... depending on how you do it of course. Calling her up in her natural form in the middle of the court-room could be hazardous to your success
So, I guess it depends on the GM. Inventive use of powers should always be rewarded.. IMHO
{what happened to the rules glossary on DDI? }
{kay.. a bit of google-fu on the rules netted me this:[sblock]Summoned Creature
A creature you summon uses these rules unless a power description says otherwise.
Allied Creature: When you a summon a creature, it counts as an ally. The power determines where it appears.
Your Defenses: The summoned creatures defenses are equal to your unenhanced defenses (powers sometimes add pluses to creature defenses)
Hit Points: The creature's max hp is equal to your bloodied value. If it dies you lose a healing surge. If no healing surges left, you take damage equal to half your bloodied value.
No healing surges: The creature starts with no healing surges, but if a power allows it to spend a healing surge you can spend one for it and it gains the benefit, but you don't.
Speed: The power determines this.
Commanding the creature: The summoned creature has no actions of its own. You spend actions to control it. You must have line of effect on it. The two of you share knowledge but not senses.
For a minor action it will: crawl, escape, fly, open or close a door or container, pick up or drop and itme, run, stand-up, shift, squeeze, or walk.
The power determines what other actions can be used for what effect. If a special command is a minor action, you still only use it once per turn,
Attacks and Checks: You make attacks and checks through the creature. It uses your unenhanced stats to make the checks.
Duration: Unless stated otherwise, the creature last until the end of the encounter or until dismisses (minor action).[/sblock]}