Hi.From my little understanding this is Alternanthera reineckii purple you speak about.
Yes i was able to make it grow emersed, without wanting.
I think my tank was lacking light intensity (and co2) for this plant, it did grow in height and out of the water. I had it for many months that way. It lost the submerged leaves at one point. Alternanthera reineckii can be grown submerged, but it is a demanding plant as far as i know, about light and probably co2 too.