Well, results so far:
ripshaw8080 suggestion for the dosbox.conf parallel1 entry works.
Now I have both printers, one USB printer and one on LPT1 going (with the redirection by the freeware program DosPrint) on Windows XP Pro.

I have just downloaded and tried the original Dosbox mb6 with Geoworks and it prints fine, but only with the same syntax as given by ripshaw8080; any other does not work. But as expected it does not print with dBase 5. Dosbox mb6 has another minor problem (as has the svn wp64 version), it locks up on exit of Geoworks (although it finally exits if one hits any key).

Prior to all this I was running Dosbox SYN Daum from Taewoong (http://ykhwong.x-y.net/) on which I could print in Geoworks, but not dBase 5 with the line in dosbox.conf: "parallel1=reallpt"
This build has no exit problems with Geoworks but I did note that Geoworks runs far faster in Dosbox MB6 than Taewoong's build.
Not sure what it all means.