I bet they would work great - Drill a few holes for plant stems and float away! Only small challenge I see is keeping them from floating around but I'd think a small peice of aluminum wire in an upside down "u" shape (one side in the cork the other in the planter cup would cure that issue) I say go for it!
I used cork in a wet spot once and it become pretty well waterlogged and starting to sink after about three months.
The standard foam riparium trellis raft lasts for years and is designed such that when you plant several stems on it it becomes covered up with foliage and just blends into the shadows. There are four trellis rafts holding the midground plants in this small tank. Even standing right in front of the tank it's hard to see the trellis rafts.