It would be great to tracks mods installed on your pins. I am always forgetting what mods I added to my pins and this would be very helpful. It probably would not require a lot of programming because you could just expand the capabilities of the maintenance log to capture mod installs. I would recommend adding a drop down field for type of maintenance (Clean&Wax, repair, battery change, mod installation, etc.) and add another field for the cost of the maintenance/mod install so you can keep track of you total investment in the pin. Tracking the total invested in a pin is a great way to continually reinforce what morons we are for getting into this hobby, Ha ha.
You might want to consider providing the ability to open a Pinside shop order and link it to one of your games to automatically enter mod log item to that game. Probably a lot more programming but you would probably get more pinsiders using the maintenance log.