Try starting in the mash or specialty grain to add your coffee flavor. Add some roasted barley to impart that coffee flavor right away. This has given me a good coffee base on many of my brews.
Next, cold extract your coffee using the following technique from Radical Brewing:
This is a way of getting very smooth coffee flavor to add to your
beer. Add 0.5 lb (0.45 kg) ground coffee to 24 ounces of cold filtered
water in a sanitized container. Allow this to sit in the refrigerator
for 24 hours, then run the mixture through a coffee filter. All or
part of this extract… may be added to your stout.
So when do you add the coffee? While secondary is the best place, but like you said in your post, that is not an option. Flameout is an option though. Add some of the cold extracted coffee during flameout or whirlpool. Depending on how much coffee you want in the beer, you can adjust how much you want to pour in. I start with pouring roughy half of the cold extract into the boil kettle. You can always add more later.
While the above is great for taste, you still want that fresh pot of coffee aroma. I stopped doing secondary fermentation on all of my beers, and I wish I could find the article that got me to stop secondary. Since I keg all of my beers, I never worried about clarity. I just chilled the beer, and dumped the first pint. I also dont mind the odd floaty occasionally showing up in my beer. I found most of my beer way coming out pretty clear from primary, so I stopped secondary all together.
The important thing with this method is when to add the coffee (or hops) to primary. I have experimented with this technique a lot, and I have found that adding the "aroma additions" is perfect right after high krausen. The fermentation has slowed enough at that time to not cause your additions to be blown out or agitated enough so the oils dont mingle with the beer. At this step, add 2-4oz of fresh ground coffee beans. Coffee loses its oils and aroma very quickly after its ground. So wait till the very last minute to grind the coffee. Coarse to drip grind would be fine for this application. While this will add aroma to the beer, it will also add more coffee flavor, since it is essentially being cold extracted.
Again, this all depends on how much coffee flavor you want. So you will need to experiment to your taste preference. The nice thing is that you can always add more of the leftover cold extracted coffee from flameout.
Also, coffee flavor and aroma will go down with age. So if its too much at first, let it sit for a while. You must be patient with beer. :-D
Hope that helps.