A Read-Only Relational Database on Google App Engine?
I don't think you're likely to find anything like that...surely not over blobstore. Because if all your data is stored in a single blob, you'd have to read the entire database into memory for any operation, and you said you can't do that.
Using the datastore as your backend is more plausible, but not much. The big issue with providing a SQLite driver there would be implementing transaction semantics, and since that's the key thing GAE takes away from you for the sake of high availability, it's hard to imagine somebody going to much trouble to write such a thing.
I have a medium size (~100mb) read-only database that I want to put on google app engine. I  could put it into the datastore, but the datastore is kind of slow, has no relational features, and has many other frustrating limitations (not going into them here).  Another option is loading all the data into memory, but I quickly hit the quota imposed by google. A final option is to use django-nonrel + djangoappengine, but I'm afraid that package is still in its infancy.
Ideally, I'd like to create a read-only sqlite database that uses a blobstore as its data source. Is this possible?