We are in a situation where we would like to have discussions with our larger clients on features that they want without exposing all ideas to the masses. This currently only seems possible by creating a separate product for each ideas portal to work from, which is not ideal.
Thank you for your idea. One option here would be to designate your large customers as partners with the "Employee/partner" portal user flag. You could then set the visibility for these ideas to Visible to employees and partners only.
Given the workaround above and other priorities, we are unlikely that implement this specific idea at this time.