There's a file in your home directory named .scala_history
that contains all of your recent REPL history. I regularly copy and paste code from this file into project source files. It's not exactly the same as showing the code for individual methods in the REPL, but it might help you accomplish the same goals.
See the comments by Paul Phillips in this issue for a discussion of some related functionality in the REPL (grouping statements in the history):
At some point I implemented the logic for this, but the real obstacle
is jline. It has enough trouble figuring out where the cursor is under
the simplest conditions. Start throwing big multiline blocks into the
history and it breaks down in tears. Would love to see this and
SI-2547 addressed by the community.
...
I expect to fix this soon too, but it depends on how well the recent
jline work goes. I implemented it long ago, and display issues are the
impediment.
Both of these comments are over two years old, so I wouldn't hold your breath.