IF you are using email as login, here's one way"
On the cart page template,
{if $login eq 'customer-1@domain1.com' or $login eq 'customer-2@domain2.com'}
<a href="COLORBOX POPUP CODE" >
{/if}
If you are not using email as login, use the userID.
You can configure a nice colorbox to auto-popup ONLY FOR the customers you want.
Is the work justified for only a few customers?