You can use my small SetClip program to do this.
This is the Opus button. (It assumes that SetClip.exe is in C:\Windows\System32; edit as appropriate after pasting it on to your toolbar.)
<?xml version="1.0"?>
<button display="both">
<label>Contents to Clipboard</label>
<tip>Copies the contents of the selected text files to the clipboard</tip>
<icon1>#clipcopy</icon1>
<function type="batch">
<instruction>@runmode hide</instruction>
<instruction>"C:\Windows\System32\SetClip.exe" -e</instruction>
<instruction>"C:\Windows\System32\SetClip.exe" -f -a {filepath$}</instruction>
</function>
</button>