Is custom field on tracking codes already in the pipeline?
No. You can answer questions like this for yourself by searching the Ideas category of the forum.
The concept seems to make little sense, however. A tracking code is already much like a custom field, though more powerful. What would your use case be? The point of tracking codes is to group income and expense items together, as for separate divisions of a business.
jeffbangquil one idea is to use Non-Inventory Items (with same codes as the Tracking codes) instead of only Tracking Codes. The Non-Inventory Items can have the Tracking Codes prefilled automatically but Non-Inventory Items and Tracking Codes work very well together and IMO such setup is more intuitive to work with, especially when recording income. Then in Non-Inventory Items you have a Description field, or just use Items Name (with code inside the name), if this is what you are looking for…
I agree with @Tut. Also a Tracking Codes can be structured to be more intuitive by using other than codes (with the exception if used in combination with Non-inventory items) so its easier to work with