Providing static/raw base templates

[OddBloke]

Hey folks,

I have an open-source Rocket application which I use as a library: some of its behaviour needs to be different for our internal usage, so it is instantiated with a pointer to a function which specifies that behaviour. This has worked perfectly for me so far as an API-only mechanism. I've also been able to add a basic templated front-end to it: I use static templates and add_raw_template so the templates are available to the library consumers.

Now, however, I would like for my internal application which wraps the open source application to be able to modify the display of the output. This is a clear use case for template inheritance: I would modify those built-in templates to have blocks for the content I want to change downstream, and then extend them in the template_dir of my internal application. This doesn't appear to be possible: templates are read from template_dir before my try_custom callback is called, meaning that the on-disk templates fail to import due to missing dependencies.

Is there any way to provide static (i.e. in-code) templates which template_dir templates can inherit from?

Thanks!