DannyMac, that's a nice pot! I don't blame you for wanting to keep the ferns in there.
Yes, I would probably "un-bury" them from the potting soil, and I'm sure a little mulch over the top would be OK. Where they grow on the ground, the rhizomes do tend to get covered with leaf litter and windblown debris [in other words: natural mulch]. They may be fine the way you have them with the rhizomes eventually growing up out of the soil, but if there aren't replacements readily available, why take the chance?.
If the mulch alone isn't sufficient to support the frond, a little creative use of inexpensive wooden skewers and maybe a little black thread could make for inconspicuous supports. You could also use plastic drinking straws or even small sticks.
On the other hand, if replacements are available if you need them, then go ahead and leave things the way they are. I really expect the ferns will do just fine, especially if you don't over water and the soil is relatively loose. Don't mulch too heavily, as you want the soil (and the rhizomes) to be able to "breathe".
Hope this helps, and good luck! Feel free to ask if you have additional questions. And don't forget to let us know how things go!
