> the org-babel-execute:scheme function looks for a :scheme header
> argument (otherwise uses the geiser-default-implementation).

You're right. I went through ob-scheme.el. org-babel-execute:scheme does
look for a :scheme header argument.

But, my original problem was different. I was looking for a way to get
rid of the "Scheme implementation: " prompt when the org file was
opened, not when the src block was executed.

When the org file is opened, some org function is being called that
ends up prompting for the Scheme implementation. I'm not sure which
function is doing that.

> This may depend on your org version, mine is 8.3beta.

I am using org release_8.3.1-933-g809a83 -- the latest version on branch
master of the git repo.