I think you can use any suffix of face_* as YAML field, then need to pass as value what is expected by the function.
I think something like this should work
output:
html_document:
theme:
version: 4
base_font:
collection:
- !expr bslib::font_google("Pacifico", local = FALSE)
- "Roboto"
- "sans-serif"
I think the !expr is required here. I don't know if this is supported or just working with current implementation. You could open an issue in bslib to ask on how to provide a font collection with R Markdown documents.