Over a week or so it'll go from about ~30% to 70+%, and shutting down the game completely does not free up the memory used.

Killing the process should infer freeing all of the memory (on the OS side) it leaked.

Using dgvoodoo, the memory used by the game slowly climbs up up to 600+ MB, and then randomly drops several hundred MB.

Is video memory counted into the total amount of memory reported?
I ask it because according to my experience mechanism of caching the video memory (at least for D3D11) at OS level lets the cache go up to some large threshold and it only gets freed after exceeding the limit. Back some time ago when I experimented with this, releasing a D3D11 object (texture or sg) didn't cause the reported video memory to decrease at all (I think it's optimized for on-the-fly Create/Release usage patterns).

I'm aware it's supposed to free up the memory on shutdown, but for whatever reason... it doesn't always properly do this. It's rather noticable as I tend to have to restart my computer every so often due to it, but never had it happen without dgVoodoo, so I'm not sure what else could be the problem.

Unsure how video memory is counted, since it reports both dedicated and shared GPU memory, although the memory usage I was reporting earlier was for the executable itself as reported by task manager. Dedicated is definitely not included as I managed to run that up to 3+ GB in a failed attempt at trying to forcefully replicate the issue with memory not freeing up on shutdown. Shared memory tends to sit at 0.0~0.2 GB no matter what.