You can force changes to items in the database while the server is live. And you can patch .SData files...
So, the easy means of doing so, create a copy of your Item.SData.
Change the rate of the lapis in the database, force the update to the server via the /reitem command, and patch in an edited copy of your Item.SData to ensure it is showing the increased rate properly.
When the event is over, simply patch back in the old and reset the rates in the database.
If you are planning to do this as a week long event, it is a bit simpler if you are using Castor's tool. During a weekly maintenance, make the changes to the .SData, patch it in and run the output query to update the database.
Do the same at your next maintenance after changing the rates back to the norm. The entire process shouldn't take but a few minutes.