Union mount

[kaaax0815]

I have following situation:

I include a JS Framework in a embed.FS
I need to add Information to a file (index.html)

The base would be a embed.FS
The overlay would be a MemMapFs

But in the overlay i would need to read the file from the base and overwrite some stuff.
Would that be possible?

What composition filesystem would fit here?

[kaaax0815]

Can easily done be implementing the fs.FS interface (only Open) function and returning the correct Open function of the underlying filesystem by checking the name