Speaking with my BJCP hat on:
To me, it's all a matter of taste. If it tastes like a fruit beer but you don't really care what style it was, and it doesn't taste Belgian, then enter it into category 20 with a base style that is NOT Belgian -- otherwise the judges might raise their eyebrows that it should have been entered as 16E or 17F. On the other hand, if it does taste Belgian in any way/shape/form, then you really have no choice but to enter it as 16E or 17F.
I'm not exactly sure what base style the New Glarus beers are supposed to have been. I've heard it said that these are lambics, but... I'm not so sure these would stand up so well to any of the Lindeman's fruit lambics, which are far more tart in my opinion. So to me, the NG's would belong more in either 16E or 20.
Yeah... perhaps category 20 really is the best fit. At least for NG. Can't say for yours without tasting it first myself. Enter it in whichever category you think it would score the highest in. And if you can't decide, just go ahead and enter it in both categories.