In order to get workout details from the past you can look (in companion app) at the activity feed of a person who attended. Workout leaders are (so far): Kevin Vermack, Patrick Smith, Gareth Harrington, Christo Toua
Select event information, then select the workout graph. This should show the numbers as well.
I contacted whatsonzwift.com to ask them if they could host the workout files until they are officially hosted on zwift. I will put an update here once I have an answer.
Update: there’s not a possibility to host them at the moment on whatsonzwift.