So, looks like the TDK/Lambda CC6-2412DF-E DC/DC converter can be wired with floating input, and is fine with either side of the output grounded, to achieve SE PSU voltage, either plus or minus. Looks useful, if I'm interpreting correctly. Not an idea I've delved into before. The addition of turn-on circuit looks essential. I'm thinking, possibly foolishly, about some -30VDC transformer coupled stuff in a 500 series rack. May play with it, and blow some stuff up. Any DC/DC PSU wizards have anything to add?
Data sheet:
http://us.tdk-lambda.com/lp/ftp/specs/CC6-xF-E_spc_en.pdf