High Frequency High-Side MOSFET Drive
I would consider using magnetic coupling for your gate drive. See http://www.infineon.com/dgdl/an-950.pdf?fileId=5546d462533600a40153559ee46511c2 for some idea how to design this.

This assumes that the length of time any one switch is on will be limited (which appears reasonable given the above circuit for the supertex parts), and the duty cycle is not too high (in a 5 level converter, it shouldn't be a problem). Suitable pulse transformers are easier to source than other isolators, the challenge I see is dealing with the miller effect and power handling of the pulse transformer.