Well, that's an option and one that some conversions use.
I don't like the idea much because I can imagine sitting comfortably in peak hour traffic on a cold, rainy night listening to great music (with no engine idling over), when the 12V system starts to fail.
So the option I have chosen is the have a small 12V battery - say 7 to 12AH sealed - which has the primary function of closing the main traction contactors, then have a DC to DC converter that runs the 12V system from the main traction batteries. The DC to DC converters are usually quite expensive but the Vogue's 600 VDC pack can be easily tapped in the middle to create two 300 VDC voltage sources. Since 300 VDC is very close to 220 VAC peak voltage, an easy source of these converters is eBay Switch Mode Power Supplies.
The only problem with these supplies is their open construction - and the small fan which I didn't want (circulating dust).
So a bit of searching "upstairs" at work found this part of a freight train radio system power supply.
A bit of work with the angle grinder and an aluminium cut-off wheel.
Then some countersunk screws, a few holes and tapped threads and another bit of 2.5mm aluminium, then a long search for sealed plastic cases the right size and...
The finished product. Splash and dust proof - no fan - just a large heat sink. They are bolted back to back with the lids on the outside to allow wiring. A single unit delivered 13.6 VDC at 10 Amps for 30 minutes without excessive heat build up. The outputs of the two power supplies will be paralleled with a suitable length of wire from each one's output to ensure reasonably equal load sharing. It's also reassuring to know that one of these can power the entire electrical system in the Vogue on that rainy, cold night. There would be a slight mis-balance in my packs but that's better than having to be towed...
Here is the latest Google SketchUp of the engine bay components. The DC-DC converters are on the right. I really did draw this before I made them (but after I found the boxes and the heat sink).