I took a second look at HTerm, and found that its FAQ (http://git.chromium.org/gitweb/?p=chromiumos/platform/assets.git;a=blob;f=chromeapps/nassh/doc/faq.txt) states:

"Clipboard read is not implemented.  Reading is a security hole you probably didn't want anyway."

Since I can't test the get-selection function and it's considered a security hole, I'm afraid I can't implement it.

This only affects get-selection, not set-selection.