Netbox has an "inventory" system for devices, where one can add parts to a device, with a manufacturer, part ID, asset tag, serial number etc.
We haven't used it so far -- for servers it's too much trouble for little gain.
I think the situation for network equipment is a bit different; routers like the MX480 have parts such as linecards that cost potentially hundreds of thousands, which we add or replace throughout their lifetime.
These are also blindspots in our CapEx planning, as they're not separate line items with purchase dates, asset tags, tasks etc. and easy to miss when we plan for our 5-year refresh cycle.
I propose that we start populating these with the "big" items, starting with MX480 parts (REs, linecards, PSUs, fantrays). We should probably also do the same for uplink modules and redundant PSUs for switches?
(To be clear, I'm not suggesting that we should track SFPs :)
BTW I don't think Netbox supports custom fields for inventory items; we should file a bug once we decide this is something we need to explore indeed.