Will - - You have figured it out - that the key does not go all the way back to the "OFF" position - - but it is not in the "ACC" position either - - so there is no drain on the battery.
As far as using the Brake Buddy - - the brakes on my 2012 Fusion hybrid work differently than the ones on the 2008 Mariner did. The brakes on the Mariner were always "powered" and so it was necessary to set the pressure very low (about 15#) for it to work properly. On the Fusion, when the key is back as far as it will go towards "OFF",and the tranmission is in Neutral, the brakes are not powered and I set the pressure on the Brake Buddy as I normally would (about 70#).
If the Brake Buddy is set properly and one drives in a normal manner, the Brake Buddy never engages (it's only set to engage in a near panic stop situation), therefore there is no drain on the battery.
If you feel more comfortable using a charge line, then by all means do it. I'm just telling you that I've towed these 2 hybrids over 30,000 miles and never had a "dead" battery problem. "To each his own."
Doug