Hi, Steve.
At this time, ET+ has these Attendee Meta field types: Single Line Text, Multiline/Paragraph Text, Radio, Checkbox, Dropdown
And none of those field types have input masks / validation.
You could request this as a new feature (or maybe search to see if the idea was already posted by someone else) at our plugins’ UserVoice page.
This allows others who are interested in that feature to easily voice their support. We frequently review suggestions there to find out which ones are popular, then we implement as many of them as we can.
If you post it / find it, feel free to link to it from here in case anyone comes across this forum thread in the future.
Another alternative is to add your own JavaScript validation. If you need some coding help, you may want to ask your developer or reference our documentation and list of known customizers.
Please let me know if you have any follow-up questions on this topic.