This improved version of Douglas Boling's DOSCLIP utility works well in DOSBox. You can copy and paste text between programs running inside DOSBox:

http://ansis.lv/dosclip/index.en.php

Do NOT be confused by the documentation, which says that this program can exchange data with the Windows clipboard! That statement is correct when DOSCLIP is run inside a Windows DOS prompt - NOT when it is used in DOSBox!! Do not be confused! This program cannot transfer data to or from the Windows clipboard when used in DOSBox. When used in DOSBox, it only exchanges data between programs running in DOSBox itself!