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But if I use twice the length of Nichrome, it won't even get slightly warmish. Heat is almost undetectable at 1300mm.
Looks like I'll have to use the 3.5 amp power supply, but then I have the other problem. It's lowest voltage is 15v. 
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Ken why is that a problem?
I'd respectfully suggest that you should repeat your earlier experiment.
If 1m of wire was piping hot, 1.3m would have to be better than just barely warm. You might have had a poor connection for the second test.
I gather Ballarat is somewhat cooler than cold at night, so the wattage of a heater will need to be somewhat more gutsy that what is needed up here in Sydney. The Dew heater I use on the LX 200 is about 28Watts, (at 12v) and it is sometimes necessary to crank the voltage up to about 18v to keep the dew away. You wont fight dew with a 12v 1Amp supply; 12 Watts is way too lite IMO. (Though an 80 or 100mm scope will need less wattage that a larger 12" scope.) As for Pulse circuits and potentiometers, pass transistors and a few other 'name dropping' devices, I believe that you will find that as long as you keep the wattage rating down around the 18-20 watt mark you should have no problems. Why?
Because (assuming good thermal contact) your scope bodies will act as massive heat sinks and so the thermal gradient across the OTA will be negligible, leaving the lens cell barely heated above ambient temperature.
which is more or less what you need. Dew point is of course what you are fighting, but there are still practical limits to the wattage you apply to a dew heater; a fan to augment the dew heater should not be overlooked as a possibility IMO.
Doug