I didn't understand the core issue, is it that due to non-alignment of the on-disk data you have to do a copy at load time, but if you could guarantee alignment of the actual blob content then you could use it directly?

Exactly, a guaranteed alignment and continuity of a blob makes it memory-mappable allowing to avoid copying the data back and forth.

On a brief background read it looks like another issue is that blobs are not necessarily stored in contiguous regions of the file. Maybe there's some clever way to combine a VFS shim and virtual tables to store page aligned blobs in a region of the same file as the main database.