First of all, it's a misconception that you need to leave Brett "something to munch on" in order to get a lot of Brett character. Brett creates much of its flavor by eating the fermentation byproducts from the original Sacc yeast (i.e., the esters and phenols that your Saison yeast produced in primary). So if your only reason to add fruit was to give Brett some added fermentables, then you can safely just let it ride as-is.
Having said that, cherries jump out at me as being an ideal fruit for this. I get a lot of cherry pie from Brett brux, and cherries also have enough dark fruit character to go with the spicy saison fermentation and the C-120's dark fruit note.
Another thought I had was apples. I'd probably go with juice concentrate or a syrup made from boiling down some fresh-pressed cider, otherwise you may just end up diluting your beer too much.