I'm with you on the value of tracking the different components of the owner's overall capital balance. While you certainly could utilize a couple of equity accounts on the balance sheet for this purpose--say, "Original Investment" and "Retained Earnings"--or something similarly titled, my preferred approach is to keep the balance sheet as simplified as possible and do the tracking and analysis separately.

For example, you might have a single "Owner's Capital" account on the balance sheet, but all of the underlying detail (e.g., original investment, subsequent investments, and draws, and the dates thereof) captured on an Excel spread.

Or, you could have multiple GL equity accounts for capturing the details directly into the GL, but have your software roll those GL accounts up into a single "Owner's Capital" number on the Balance Sheet.

Don't think that if you set up a "Retained Earnings" account on a Sole Prop balance sheet, that the Accounting Police are gonna haul you out back and give you a blindfold and a last cigarette. Feel free to set up your accounting in whatever manner helps you and/or the owner understand, analyze, and run the biz better.