My Garmin 705 seems to run just fine with an external battery attached to it. If I attach the external battery with the Garmin turned off, it turns on. If I attach the external battery with the Garmin turned on, it continues to function. An icon does change indicating that the Garmin is running on external power. If I remove the extenal battery, I receive a message that the external battery has been removed and I'm ask to push "enter" but the Garmin continues to function.
I've mounted the external battery on my bike so I think I'm good to go for the double century this weekend.
The reason I wanted to use the Dinotte battery is that it is huge compared to the AA batteries so I assume it will last much longer. Additionally, the Dinotte battery is sealed and has attachments to easily mount it to my bike.
If anyone cares, I'll report back how it works this Saturday.