Just bought a 2.8 turbo to go in my F20 so I will have the same problem. I checked Denso's online catalogue for glow plugs and found part number DG-216 as a 11V alternative for DG-223 which is the standard 7V plug for this engine. All dimemsions, threads, etc are identical, it is even a Daihatsu/Toyota fitment. I reckon that's going to be the one to use - at least that's the road I'm going down when I get round to it in the next few weeks. I intend to wire up a momentary push switch on the dash and a 70A relay to switch the glow plugs and use my brain as a timer and temperature sensor!! Since the engine isn't running when the glow plugs are used the battery voltage won't be more than about 12V and so long as the cable to the glow plugs isn't overly thick there should be another 1V or so lost there which makes about 11V. I might do the maths and come up with a proper figure for the likely voltage drop but I haven't got the time right now.

Hope this is of some use, I'll let you know how it works out.
Regards - Alastair.