the ir detecting devices are usually designed for one specific wavelength. I have no idea which one the wii sensor bar is using.
If you would have 5 led's in serie with a 3V forward voltage, than those would need at least 15V before any current could flow.
So, I think the forward voltage is lower.
I don't think the wii ir camera is looking for a specific number of led's.
Code:
http://letsmakerobots.com/node/7752
The camera in the wiimote seems to be able to track 4 separate ir sources only.
You could try a second motion sensor right above the normal one and see if it makes a difference. I mean, it's not like they are so expensive. Most non brand sensor bar's even have an option to insert batteries. While it's not ideal for every day use, it might be good to see if accuracy and range improve. Afterwards, you can always supply the sensor bar from a 12V adapter, or mod it so that works from the 5V supply of an usb port.