Thanks for the pointers ManiacGT.
Searching for the terminals to jump start under the hood, I came upon a video showing where the BMW terminals to the battery under the hood and a portable battery jump starter unit that can both act as a backup battery, trickle charger, and jump starter all in one.
I needed a portable solution because I'm not near any electrical outlet. Here's the video and name of the charger.
DieHard Platinum Microprocessor-controlled Battery Charger Engine Starter
Hope this helps anyone else in the same predicament as me. Now I just have to check if there's enough space to fit this backup battery to trickle charge with the hood closed.
Keep up the fantastic forum support!