I decided to go with the DIY on-board isolation transformer solution as well but in a primary - secondary - secondary - primary configuration. Hardly any voltage drop or heat generation so the surgery seems to be a success. Also added a IEC male socket, fuse holder and a mains earth switch. Switched on I get about ~17V between device ground and mains earth, turned off a couple. Its a basic 2Mhz model and came with firmware revision 3. Apart from SIN, SQR, and TRI it has RAMP, NRAMP, PULSE, 4-PULSE, 8-PULSE, STEP and DC selectable wave forms. There's a FOLLOWING option in MENU which let's you select AMPL, OFFS, DUTY and FREQ for CH1=CH2. Have to work out what this does exactly, the manual is for an older version without this option. A big thanks to Classical for the isolation suggestion.
Cheers,
John