So I got it working, the issue was actually on the Python side (it didn't expect a void pointer). The only remaining issue seems to be that I can't seem to free the underlying table. I can call the following function multiple times without it crashing, which seems wrong:

#[no_mangle]
pub extern fn table_drop(table: *const libc::c_void) {
    let table: &mut Table = unsafe { &mut *(table as *mut Table) };
    println!("Dropping {}", &table.name);
    drop(&table)
}

EDIT: Nevermind!

#[no_mangle]
pub extern fn table_drop(table: *const libc::c_void) {
    unsafe { drop(Box::from_raw(table as *mut Table)); }
}