Bit annoying that the backlight won't turn off when it's using external power, although it does stay off if you turn the backlight to 'off' in the settings.This does mean that it never comes on, though, so not ideal for a 24hr.
Unless I'm missing something I can't see this as a problem and saying that the backlight then never comes on is true but a bit misleading, as you would go to the exact same display setting page that you previously used to select backlight 'off' and then switch the backlight back 'on' when you require it during the hours of darkness .
If you didn't want to bother doing this and wanted to not mess with the display settings, I'd suggest a 2600mAh battery pack will charge your Garmin battery from empty to full charge 3 times (Garmin battery capacity is 700mA). Assuming you started with a fully charged Garmin battery as well - each 'full charge' would have to last 6 hrs with the backlight permanently on to cover a 24 hr ride.
One poster here suggests he was easily getting 12hrs with the backlight permanently on a lowish brightness setting, so with a 2600mAh external battery, 48hrs worth of power should be achievable.
LinkThis would only be the case if you had the external battery permanently connected which is unneccesary. As you know when not connected the Garmin display backlight would time out automatically according to your display settings.
2600mA external battery is about as small capacity as you can get from Ebay what was the mAh capacity of the one you bought?