What kind of buttercream is it? I'm guessing it doesn't need refrigeration because of the question. The sugar will act as a preservative, but so will the sugar in the buttercream. I wouldn't say indefinitely, but you have a few days.
Have you ever tried gourmet jams? I have found them to be less sweet and candy-like than the cheaper and grocery brands. I use them in buttercreams, as fillings, and in pastries. I have had to go through a few duds to find the right ones. Our farmers markets and Penn Dutch markets have great jams and honey.
I just bought some frozen raspberries last week and will be making puree in a few days. I'm still torn between adding sugar and leaving them alone. But because it will be for museum work in meringues, I'm thinking sugar. I'm also considering a thickener. This is one of my commissioned contracts, so it must be authentic to a period in history. I just don't know what period yet.
Good luck on your project. I'm sure it will turn out great. Susan