i'm not sure I would go in the direction of adding sweetness builders or unfermentables. I agree that raspberry can be quite tart, but it can also deliver a good dose of tannin if unstrained fruit solids are added to the beer. The seeds are the source.
I'm curious if either of you had the opportunity to taste the base beer before adding the fruit or flavoring? Since its unlikely that the base beer was too tart to begin with, I'll assume that all the tartness came from the fruit.
On the thought of increasing the 'sweetness' perception, an easy adjustment could be to reduce the bittering level. Increasing the mash temp would also be an option. If the fruit tartness is the primary cause of the overall beer perception, then another option could be to add an alkaline buffer to the finished beer. Since there are apparently free acids in the beer, this is one situation where chalk could be an effective agent for reducing excessive acidity. Recall that chalk doesn't work well in the mash since the acids there are too weak. I'd suggest pouring a glass full of beer and adding measured doses of chalk until the level of tartness meets your taste goals. Once you've figured out the amount, you can scale it up and add it to the keg.