You really won't be able to, unfortunately. While your email alias considers everyone@ourURL.com to be tens, hundreds, thousands, etc, of people -- Marketo only sees it as one person. So like you said, the second someone opens it, you're going to have a 100% open rate (assuming you only send it to that alias).
I know you said you don't want to manage another spreadsheet, but does your HR department or somewhere else not already do this? In a previous job I simply used a sheet that the HR dept was already maintaining and would update the list prior to internal sends.