Bypassing the regulator won't work since this is a shunt regulator, which is only active when the voltage gets too high... It doesn't do anything at idle anyway... The core of the problem is that the stator coils don't have enough windings to generate 12V AC at idle with a 45 Watt load or so...
There's nothing you can do about it, apart from making your own stator coils with a few extra windings, but then you would also have to find (or build) a regulator which can stand the extra power at higher rpms....
Oh, and if you're really adventurous and skilled in electronics you can rewire everything to a DC-only configuration, which also means you'll have to design a charging circuit which uses all the stator coils for charging the battery, instead of only the smaller coil..
I've been thinking about these solutions for a while myself, but solving this small problem is just not worth the trouble it will take... Trust me, I make a living designing and building custom electronics...
Best solution : adjust the idle rpm somewhat higher (but not so high that the clutch will engage of course), your lights will be brighter, and your engine will be happier since the oil pump will also give more flow of that precious juice which keeps it healthy