Hi Dave -- have you tried setting up a Metadata Preset and adding it to your Import settings? You can see, create, and eidt Metadata Presets via "Aperture→Presets→Metadata". I always use one. For my shots, I include the following fields:
{IPTC Contact} -- all of them.
{IPTC Content} -- Keywords. For adding keywords at import. In practice, rarely used.
{IPTC Status} -- Copyright ("©MyName") and Usage Terms.
This is the text I use for Usage Terms:
"No Usage Rights Of Any Kind Granted Without Written Authorization From MyName"
You can type the copyright symbol on a Mac by "{Option}+g".
In the Import dialog, click the dropdown button at the top right "Import Settings" and add a section for Metadata. In the Metadata section, click the dropdown and select the Metadata Preset you have set up.
If you include the Caption field, but leave it blank, it should (afaik) come up blank for each import -- allowing you to add a caption to be applied to all files being imported.
It is worth examining the other Import settings to see if any are of use to you.
I think that's what you are asking. If not, ask more.