Here's my Very High Tech implementation of the battery remote on a ACK-DC40 clone dummy battery.
Main point is that (on some cameras at least) it's not too hard to sneak the wire out on the side of the battery. The same approach would work with a real battery, if you masked the original terminal first.
There's more room on the SD card side.
To trigger, you just need to go through a resistor to ground, provided here by the HDMI out.