Observable engineer here. I haven't looked into Malloy much, but Framework's data loaders are very flexible. If you can write a script or binary that uses Malloy and writes to stdout, it can be a data loader. For example, although we use SQL a lot in our internal usage of Framework, Framework doesn't actually have any specific SQL support. We just use our database's normal bindings.
Can you link to the "data loader" API or perhaps even a "data loader" example implementation for something similar to malloy, duckdb, or any other DB/SQL data source/provider?
The docs for data loaders are here: https://observablehq.com/framework/loaders. The simple version is they are simply programs that write their output to standard out. Very Unixey. When those programs are referenced in client side parts of the JS, they are reactively run when in development, and prebuilt for deployment.
I don't think we have any full examples of using a database yet, but we have written a bit about using DuckDB via its Node bindings here: https://observablehq.com/framework/lib/duckdb
I imagine that either Malloy's CLI or its Python bindings would fit very well here.