I don't think this would be possible, i'm not an admin or anything but i can sorta figure out why they haven't introduced and don't have plans to.
1. Diamondfire is a smaller server than other servers like Hypixel that make loads of money and they want to still earn some revenue from the server rather than people put up a plot ad as a overlord and give people with no rank a overlord plot.
2. This could easily break the servers and cost quite a bit of fps, depending on the strength of the server as they would have to monetise who is the main owner (person who rented the plot to them) and the 2nd owner (person who rents the plot), and they would have to implement a system to take back a plot which i imagine would be quite the struggle on fps and also roll back recent changes.
3. People could abuse the system and do a plot ad for "Free overlord plot rent" and easily once someone has made a full game cancel the rent and just take the game, so they would have to make a roll back changes when you cancel a plot rent which could date back months of time, and would have a lot of data to store in the server considering the amount this server has increased in code blocks and data storing for probably over 100,000 plots. As mentioned in the last reason, it would take up a lot of data and cause quite a bit of server lag.
So i'm not so sure they will be adding this soon or in the future.
:)