Thanks to Ronald Regensburg at www.emaculation.com, there’s now a workaround for this problem of the apparently empty clipboard when copying from SD in SheepShaver:
If you keep a SimpleText window open, you can copy from SD and then paste the copied text into SimpleText. If you then select the text in SimpleText and copy it again (or, more efficiently, cut it from SimpleText), you can then paste it into SD (or anywhere else). The pasted text won’t be styled, but it’s better than nothing.
Presumably an AppleScript could perform the task of copying from SD, pasting into SimpleText, and then cutting the text from SimpleText, so that it could be pasted again. (I haven’t checked whether SimpleText is scriptable, but any scriptable text editor should do the job.)
Also, this problem does not occur under SheepShaver for Windows. So perhaps those of us who write scripts for OS 8/9 should do our scripting under Windows…?