Couple things you can do, since the operating volatge of the headlights are known too.
12Vdc hooked to a batt = 53 watt draw = ~ 4 amps continous.
How bright did the light shine?

You can hook an Amp meter in series between the DC out and the headlight, make sure you're on the high Amp plugs and setting.

While running this you CAN get an Amp off the AC side, but going into headlight would be best.

Got a 2nd meter? you could while all this is running hook a 2nd meter across the headlight prongs, set for 20 Volts or higher DC and get a reading while unit is running.

These are quick and dirty and won't give you the highest output load of your 'mill but will get you some numbers.

So now what...?  P=volts^2/Resitance=IR^2....

The resistance of your headlamps is highly non-linear. Measuring the resistance cold with a multimeter doesn't tell you much; the resistance will increase considerably as they start to give off light (and generate heat).  

Best approach in your situation is to find a low value, high power resistor (put in series with your lamps), and measure the voltage across that to calculate the current. Nought point one ohms (0.1 R) at 5 or 10 watts would probably do. A finger calibrated in degree's C could come in handy.

The ammeter is an easy fix.  Will need one soon anyway.

For example, when you are testing "open-circuit", then no current is flowing, so the voltage developed at the rectifiers is unaffected by current:

I=V/R  
...in the open-circuit case, the resistance between the leads out of the rectifiers is nearly infinite (air, or a very high resistance circuit in the volt-meter) so the current is zero.

If you plug a 1-ohm resistor across a car battery, then the current flowing from the battery, through the resistor, back to the battery can be worked out:

I= 12v / 1 ohm = 12 amps    

I= 12v / 0.1 ohm = 120 amps

Many things that provide you with voltage, such as car batteries and alternators, like the one you've built, are actually affected by the current when it flows.  Put a larger current load on the battery, and you will be able to see its voltage drop.  For example, if the voltage was 12v when the battery was unused, and putting a 1 ohm resistance across gives you 12 amps, then putting a 0.1 ohm load across it will not get you 120 amps.  Not because Ohm's law changed, it's because the voltage that the battery can supply is limited.

I = 10v / 0.1 ohm = 100 amps
...is more what you would expect by pulling 100 amps out of a car battery.  You can try this and check with a voltmeter across the battery terminals.

Your alternator is also affected in this way, not for the same reason.  The battery works on chemistry, the alternator works on alternating magnetic fields.  In both of these devices there is an action and a reaction.  They make current flow, but the current they can produce is limited by a reaction inside them.

Most people aren't used to this because mains 120v power from the grid is rarely affected by this.  The effect is actually there, but it's hard to notice because it's so small, and it has more to do with the wiring in your house than anything happening at the electric utility.

The graph of voltage you plotted for us earlier looks good.  Now that you've mentioned that you let things warm up, that may explain the "step" in your graph.  Once electric equipment heats up, the resistance to electricity also increases.  The magnetic permeability of many materials are also affected.  When you start tying battery charge tests, you'll also find that the voltage of the batteries rise up and up.  There's not a lot you can do to prevent that without investing in battery charge control equipment, so I wouldn't worry about it.  If you use enough of the forklift batteries, the alternator won't over-charge them anyway.

The alternator also looks well suited to the speed of a VAWT, but you will not know until you mate the two whether they work together.  To be honest, going the distance to carefully load test the alternator with a battery setup will not help figure this out much, so you could just put them all up and see what happens!

Since you say you've observed some softening of the resin in your previous tests, then I'd still encourage some bench tests to monitor that.  Running the alternator under battery charging load will heat it up a lot more than before.  Did you post-cure the resin after it first hardened?