But isn't rasterization necessary for this deception to work? Unless there's some magical scanner out there that's outputting vector images.

Whether leaving as vector works depends on how much your recipients care to investigate.

The above convert command produced credible output only 6KB larger than the input PDF in my limited testing.

When I tested using only IM I was getting 3x file size. I will further test your command to see how this improves the current situation, thanks!!

But that in itself is a tell.

Anyone unsophisticated enough to require you to print and handwrite a signature is also unsophisticated enough to not realize that the PDF size is a tell.

The whole purpose is to make a new PDF that looks like it was scanned. The file size differing from the original is a feature, not a bug.