Lot's have tried, many batteries have died.
I saw these figure awhile back on the forum not sure if it was for the early or later models. Both have limited output.
19.5 watts or less at 1500 rpm
143 watts or more at 5000 rpm
260watts or less at 8000rpm
The wise Qwerty once said
"I've crunched the numbers for both new and old TWs. The secret to a healthy electrical system on a bike like the TW is to maintain the factory balance of charging system outlet vs. power draw. There is no regulator per se like on a car or truck. Generally speaking, such systems put out just a little more than the bike needs to run. Once the battery is charged there is just a bit of overload going into the battery that is disipated as heat.
On those TWs 2000 and older fit the new H4 headlight housing and a "bi-xenon" H4-to-HID conversion. It will probably be necessary to add a regulator and small second battery to provide enough voltage at idle to maintain the plasma arc of an HID at idle since the headlight runs directly off the stator on the early models. If the other lights are converted to LED either use resistors or add auxilary LEDs to match the total draw of the original bulbs. Failure to do so would likely result in an over-charged and ruined battery.
On the 2001 and newer there are two options. The first is to switch to a "bi-xenon" H4-to-HID and add 35 watts worth of incandescent auxilary lighting. Most incandescent clearance lights draw about 3 watts. The second option is to mount dual headlights that accept "bi-xenon" HID conversions and convert all other lights to LED of equal effective luminousity to the originals. There will still be enough watts for a few extra LED brake/stop/tail lights. With the 2nd option one headlight can be switched to provide an additional 35-40 watts for electronics or heated gear.
On "bi-xenon" HID conversions: Cheap HID conversions are HID on either high or low bean and conventional halogens on the other. It is impossible to maintain electrical balance when flipping the dimmer causes a 20-25 watt change in current draw, or more. Also, with some of these conversions it takes a second or two for the HID to put out light. Hit the dimmer, and the halogen dies a second or two before the HID makes light. Kind of scary in traffic at night. "Bi-xenon" HID conversions use a single HID plasma arc to generate light output and use a servo to change the position of the plasma arc to replicate the position of the two filaments in an H4 bulb. If you are going to do the H4 conversion bi-xenon is the way to go."