First: the wizard in the strip is WAY out of line. That's dominate monster not suggestion.
Second, a few notes on Pielorinho's idea:
1. It's already somewhat incorporated into the description of the spell iteslf. "That pool of acid is really water, wouldn't it be nice to take a refeshing dip?" Both lines are part of the spell, but the first is describing the situation deceptively (though, as I interpret the spell, not
obviously deceptively--the "water" has to be the kind of acid that actually looks like water). That seems to very strongly imply that "you're outnumbered and we've killed two of you; you should turn traitor so we'll let
you live" is acceptable. The first part describes the relevant situation (accurately in this case). The second describes a suggested action. No problem.
2. The notion of using a skill check to make certain courses of action seem more reasonable seems like common sense. Out of combat (or prior to it), at least, diplomacy could well convince the NPCs that you would accept their surrender. If you then, during combat, suggest "you're outnumbered and outclassed; you should surrender--it's the only way to save your neck," it is a far more reasonable suggestion than if you spent the pre-combat rounds trying to intimidate them by boasting about how you'd dig their intestines out with a blunt spoon and feed them to the wild beasts.
Similarly, the suggestion "your master will assume you spilled your guts to us and will kill you too if we lose; your best hope is that we defeat your master so you should tell us the location of the traps in the secret passage" is much more reaonable if your party has displayed skill in the combat where the NPC was captured and has intimidated/bluffed him enough that he thinks you'll kill or torture him if he doesn't talk.
Thus skill checks and even attack rolls should rightly figure into the reasonableness of the suggestion. As a DM, you have a choice of using skill checks to determine whether you convince the NPC you'll accept his surrender/kill him if he doesn't talk or ignoring skill checks and arbitrarily deciding. If you use dice for social interaction, however, changing an NPC's view of you and his circumstances is what those skills do. The second choice as a DM is whether to apply reasonableness to the roll. If you don't, you get some awfully funny results, however. (See OotS, for instance). But, if you use dice for interactions AND apply reasonableness modifiers to the save, then I think you've already decided that Piehlorhino's examples work in principle and the only question then is how many actions they take and when the setup will work.
My take on this is that a free action preface to a suggestion will only affect the reasonableness DC modifier if it is readily apparent and/or the target believes it to be true already. Thus "you're outnumbered and outclassed; you should surrender" gets a reasonableness penalty to the save if the caster's side is clearly superior and simply doesn't work at all if the caster's side appears inferior. (Being based on an obviously false premise, it's absolutely unreasonable and doesn't work. No save necessary).
The second example is obviously outside of combat thus the success or failure preceeding bluff will determine whether the suggestion merits a save bonus (no, I don't think the boss did tell you it's an emergency) or a save penalty (hmm, the boss told them I'd do it so he'll be mad if I don't).
Trying to change what a target believes in combat will require careful setup (a simple one-line bluff will state the grounds to consider the suggested course of action reasonable or unreasonable, but probably won't significantly change his extant beliefs and perceptions; on the other hand, a series of actions--free or otherwise--over several rounds that is consistent with the grounds you are suggesting could trick the NPC), but could affect the save dramatically given the right situation.
Pielorinho said:
As a related question, do folks see skills such as diplomacy and bluff as coming into play in the casting of Suggestion? That is, can you use diplomacy to make an unreasonable request sound more reasonable? ("Listen, we've got your side outnumbered and we've already killed two of your compatriots; I suggest that you turn traitor and start working on our side, helping us kill the remaining schmoes!")
Can you use bluff in a similar fashion? ("I know you're not supposed to tell anyone the password to the meeting-chamber, but your boss said it was an emergency and that he ordered you to give it to us; I suggest that you go ahead and tell us the password to the meeting-chamber.")
Daniel