Yes. We do something very like this where OOO are blocked by default, but any user can chose to allow their OOO to go out or not every time they create one. This was done due to some security concerns around OOO messages and forces the user to make the conscious decision that their OOO is allowed out of the company each time they draft an OOO message.
I prefer this approach since this way we do not have to sync and maintain a list. It also keeps users from trying to work around the OOO block in unsavory ways since the workaround is available to them if they need it anytime.
Under content filtering, create an OOO blocking dictionary. Set a header search entry for "Out of Office" to weight 100
Pick another string for the user to include at the end of the message that looks like the end of a signature, like "~~~~~" (or whatever works for you)
Set this as a body search string at weight -100 in the dictionary (this is the string to allow OOO to go external)
Set the rule to block on weight of 100
1 - Normal OOO messages from Outlook to the outside will be blocked
2 - OOO messages with the bypass string included by the user will be allowed to go out.
Communicate the byass method to users. Done!
Note - If you serve other countries, they will need to provide you their language wording for additional dictionary entries (also weight 100) to be blocked.