I am using a pot to tune a 5v Voltaic regulated 5 v battery to 4.3 v.
Please provide a schematic. If you are simply using the wiper of a pot and setting the voltage by adjusting the pot and measuring wiper to ground "open circuit" then you are going to get a big surprise when the camera turns on and trys to draw upwards of 2 amps (albeit briefly).
When there are batteries in the battery compartment the cameras power down the instant I plug it in.
This is normal - inserting the plug into the jack disconnects the batteries.
When there are no batteries in the compartment it won't power up using the DC-in connection this way.
See my first comments above.
Is using an external battery wired to proxies the only way to extend battery life in these models?
Proxies ? You mean so called "fake batteries" machined to the same shape as AA cells but wired to accept external power?
If so , then "no". That's not the only way to extend camera shooting time. See my first comment in this post.