I'm very much a full-name person as I don't believe in limiting choices, and I think nickname-type names can pigeon-hole people in a world which is all-too often based on first impressions.
However.... I dunno, I sort of think you could maybe get away with it with Joni. Then again, maybe not. The fact that it ends in an 'i' tends to make it seem a little insubstantial, I guess.
Hmm, not much help, am I?
I think you'll have to go with your gut on this one.
I would add though, that I agree with Stellan re Joan, and I do think if you called a DD Joan, you will be ahead of a curve.
I think names like Joan, Judith, Jean, etc, are going to become much more current and pleasing to our ear as we inevitably move away from the frilly Milly, Molly, Tilly, Evie, Daisy Poppy, Ellie, Ella, Isabella type names.