Some larger-polyped corals are naturally found on soft substrates. Elegance coral, Trachs, Rose Coral and Fungia/Cycloseris are examples. Rose coral is a brownish Atlantic coral that looks very similar to a Trach and is legally collected. The cone base on Elegance, Trachs, and Roses allow them to sit in soft substrates without sinking in. The plates (Fungia, Cycloseris) are able to stay on top if the current isn't too strong. Many Elegance coral and Trachs are collected from deep water and will probably find the lighting for seagrass too bright. I keep my red Trach and red Lobos tucked under rock and in the back corners of the tank where they are out of direct light and they do fine.
Other large-polyped coral will do fine in a seagrass tank, as long as they have enough support not to sink into the soft substrate. Before you put in the sand you can put in some rubble piles of small rock and shell to support these. I have a few Favia, Favites, and Lobophyllia, these all do well on or near the sand.
One coral that has done very poorly on sand for me is Hydnophora micronos. Whenever I've put it on the sand it has had a kind of recession start that can only be stopped by fragmenting off the diseased part.
The main thing is, can the coral shed the sediments? Not all LPS can. On the other hand, some SPS have done just fine for me right on the sand.