And their demo code looks like:

    #include "sqlite3.h"
             
    #ifdef USE_LOCAL_DATABASE
    #define DATABASE "/var/databases/invoice.sqlite"
    #else
    #define DATABASE "sqlitecloud://username@password:localhost/invoice.sqlite"
    #endif
            
    int main (int argc, char **argv) {
      sqlite3 *db;
      rc = sqlite3_open(DATABASE, &db);
    }

They (in theory) provide:

> SQLite Cloud provides an open-source C library that is 100% source code compatible(1) with SQLite (based on our official SQLiteCloud C SDK).

Why not ship a sqlite3_cloud.h file? You're already using a conditional macro anyway. I assume you could also swap this version in when compiling something like Python, but the friction for a company to use this is really high. Especially when SQLite is designed for a protocol-less, local, experience. I think the fly.io method of moving an application+sqlite3 database both to the same cloud node is probably the better choice.

Or, it might be possible to do something with SQLite's virtual FS module to keep it all vanilla SQLite and still store the full database remotely.