Pretty sure I've seen it done. But also pretty sure it looks like what you think it would. Kind of half-done. If you know what I mean.
Nothing wrong with the concept. Covering just the front where most people sit most of the time would be fine, and a lot of kids and passengers in general don't mind being out in the sun on a nice day anyway.
But I think one big issue with the fit arises when the main hoop is in a different position than the one for the basic rollbar was. In other words, is it a single hoop still? Is it farther forward or back? Are the stringers and kickers in convenient locations that don't mess with the bikini? Stuff like that.
And if, like many, the bikini actually has a sort of a "seam" where it's expected to cross over the main hoop, it will look even worse if the hoop is in a different position than it would if it was just a plain bikini with just straps, but no actual "seam" or crease where it's expected to cross the bar.
Bottom line though, is get the cage system that you want, then try the bikini to see what you and the family think. If it looks alright, go for it. If not, it's easy enough to get a full bikini or have one made to fit your particular cage style.
Good luck. Post pics either way, so others can see how it turns out.
Paul