Being able to do this over USB is unlikely but not impossible.
My Nikon D750 has
7-9V 2.5A stamped on the bottom of it. That's a draw between 18W and 22W.
This is significant because you can buy USB step-up converters to get to 9V. The problem is most of these (one example) are limited in terms of current output to around ~5W... And 7W before they start to melt.
I'm certain you'd start to get supply issues at the USB end too past a point.
So short of chaining together multiple supplies, this probably isn't going to be an option for a full-on DSLR. It might be an option for a lower-power camera especially if you can disable the screen, etc.
If you can do the timing internally (many DSLRs can) is a battery grip available? I'm not sure how many shots you're planning but in my experience the timer doesn't use that much power. I get weeks of standaby from mine. I could double that with a battery grip. Perhaps more with higher density batteries.
What's killing it for you is being hooked into a USB interface. Assuming you have good standby power usage, an external timer would be a better idea than leaving it on USB indefinitely.
A separate power supply is by far the better option but if you're out of range of an AC supply, a car battery can easily supply currents like 2.5A through a cheap step-down transformer. This one can pump out 8A. 12A if you improve the heatsink.
More than enough to power the camera. Add a 5V supply power supply and you can power the Raspi too.
The great thing about picking a 12V lead-acid supply setup is it's used everywhere and rural demands mean there are solutions for keeping batteries trickle-charged via solar (or mini wind/water turbines, etc). Indefinitely.
The problem now becomes keeping your camera protected. I'm not an electrical engineer (I'm just enthusiastic and well-caffeinated) so it might be ill-advised to stick something like that into your camera without additional smoothing and/or additional voltage regulation.
If you're uncomfortable with this sort of stuff, I'd aim to find a local electrical engineer. The parts cost for something like this (past the battery) really are negligible.