I'm not entirely sure if you can just generally outline dialogue in the way you described...dialogue is a very delicate, situationally-specific story element. Making sure it's "well-written" and engaging is usually something you have to work out and refine while you're crafting it, not something you can decide on ahead of time. ^^;
I think creating 'objectives' like @IndigoShirtProd explained is about as much as you can do to prepare; as for the rest, if you're not yet sure how the words are gonna come across, you kinda just have to put them down and then analyze them; figure out what they're missing and how they can be improved. Write bad stuff first, then make it better; and along the way you'll learn general guidelines for fixing things much more easily as you go.
For example, you said:
Experience has taught me that this is probably because you're
A) Writing dialogue that you don't actually believe in (i.e. things you think your characters 'should' say, rather than things you want them to say)
and/or
B) Writing dialogue that feels artificial/transparent (i.e. it's obviously meant to push the plot forward, rather than sound organic or situationally appropriate)
I think both of these things happen to everyone occasionally-- at the very least, in my work, I find them impossible to avoid completely. ^^; But you definitely want the lines that feel like this to be a tiny minority.
In the case of A, I would consider adjusting my objectives. Maybe I'm trying to create a characterization that doesn't actually fit the characters' behavior thus far, and I should change the dialogue (and possibly the plot) in order to better reflect their reality.
It's VERY easy to get so caught up in a character's 'assigned role' in the story that what you have them do (when you're enjoying using them as a story element) stops matching up with what you have them say (when you're trying to 'accurately' portray them as a character).
It's like the #1 most common mistake I see professional writers make; there are so many people I'd like to shake and scream at them "IF YOUR ORIGINAL PLAN FOR THIS CHARACTER ISN'T WORKING; CHANGE YOUR PLANS!" An actually enjoyable character is always better than a shallow puppet, and a good story should service the former, not the latter!
Now in the case of B, I would recommend actually using less dialogue. ^^; This is a good deal of what people mean when they say 'show don't tell'-- you need to know when to let your characters shut up.
I think it's actually a little harder to do this in comics than in prose, since you can't always use narrator's privilege to just tell the reader what's going on. You have to come up with artful visuals that give the viewer the information they need, and put the work into drawing them.
But I think it's always worth the effort, especially if it helps you cut out a lot of clunky expositional dialogue.
"This is A-kun, my best friend. We always meet up after school to discuss our evil plans!"
[SHOW the characters leaving school and meeting up-->] "So, A-kun, did you think about what I said...?" [--> start a conversation about their evil plans that SHOWS their friendship]
"Overusing the magical artifact must have caused these negative side effects! We know this is what happens because lore, lore, lore; blah blah blah"
"So you think the magical artifact will solve everything, hm? Be careful what you wish for..." [--> SHOW the character using the power and experiencing negative side effects] [--> SHOW the character seeking to relieve the side effects, and thus learning why they can't avoid them]
...Basically, give your readers some credit and leave them some space to fill in the blanks. Sometimes actual events with a few bits of well-placed dialogue can deliver information much more naturally and enjoyably than one character standing up and spelling everything out.
Even knowing these sorts of strategies ahead of time won't prevent you from ever writing icky dialogue, though-- as I explained, they're tools to help you fix things.
TL;DR, if I were really intent on meeting a specific page count and having really efficient dialogue, I would start by simply writing whatever I want. Pages and pages of back-and-forth garbage, if I felt like it...and then, I would use strategies like the above to trim it down and fine-tune it into what I need for the story.
