I think we figured out a way to do it.

I created a batch file called gps_time.bat with the following contents:
c:\windows\net time \\gpsclock /set /yes

Checked the "Close on Exit" property

Changed our VFP calling routine to

run /n7 x:\foxv\startup\gps_time.bat

Hey, presto - no DOS window!

If anyone has a neater solution I would be deligheted to hear from you