I just received a pot of marsilea quadrifolia which has been grown emeresed. I was surprised to see the plant over 8" tall. Will it revert to a shorter submersed form? What if I trimmed it down a little?
If you feel brave enough then cut all the leaves off and just plant the rhizomes under the surface of the substrate. Otherwise plant it with the emersed leaves and new submersed leaves will form. When they do form then cut off the emersed leaves as they will soon start to die off and attract algae.
BTW if you want to bulk your plants up quickly before planting them then they grow very quickly emersed in a pot of compost sitting in water. I've got a whole seed tray of emersed grown M.crenata that I was planning to plant in a tank as a carpet as once submersed it doesn't grow anywhere near as fast for me.
Sounds like an excellent plan, but it'll be a tall carpet until you submerge it! I forgot to say last time that the submersed foliage is on much shorter 'stems' and usually single lobes, rather than the 4 lobed fronds when grown emersed.
From what it sounds like you're planning, split it apart and plant it emersed, let it grow emersed in your tank and establish and spread and then cut all the emersed growth off, flood and wait for the submersed leaves to form. Take plenty of pictures so we can see how things go!