...But is there a store inventory (not user inventory, only store) comparison tool for these digital storefronts yet? I was going through and converting stuff on my Steam wishlist to my GOG wishlist where I could, and couldn't help thinking this would have helped me avoid having to do that to begin with.

I've been gradually trying to convert most of my library to GOG and DRM-free where possible, which is kind of easy with the bigger publishers (they generally don't come here till a little while down the line, if they ever do), but much tougher with the smaller developers.

I know this has probably been asked about before, however most of my search results only produced results related to user inventories instead of store inventories, and for all I know, in the intervening time since last asked and now, maybe someone's produced a tool like this. =O

Have you heard of Project Ascension (still in development)?

While extremely clumsy, you could try to utilise IsThereAnyDeal.com for this purpose. By searching for a game you're wondering about, you can see which stores sell the game (and their current prices, which is the purpose for the site).

Of course, a single game may be recorded under several titles (searching for "morrowind" got me - among others - "The Elder Scrolls III: Morrowind® GOTY Edition" at GOG, DLGamer, and a couple others, "The Elder Scrolls III: Morrowind" at Steam, and "The Elder Scrolls III: Morrowind® Game of the Year Edition" at Humble Store, GreenMan Gaming, and Uplay) and each slightly different title is its own entry as far as the site is concerned.