Could Coercive Portal be sideboard tech for Workshop decks in the mirror or against other creature decks?

Absolutely. It's not a staple of the deck, but people occasionally use the card in just the way you described.

Maybe a more experienced shops player could elaborate on which matchups it works best in / what cards it works best with

So it's a cool card, and I owned a foil set of them at one point because I felt like it had some merits, but the biggest problem with it is the time that it takes to gain a real advantage.

If your opponent opens on an aggressive hand, you want to counter their aggression. If your opponent had an aggressive opener, would you rather have a four mana artifact that drew you cards, or a three and two life artifact like Phyrexian Metamorph? Would you prefer a Pithing Needle to shut off opposing Ballistas, Wastelands, Overseers, perhaps? Immediate effects that instantly impact your opponent's ability to shut you down, or wriggle out of your dominant board state, are better now. You will likely draw between 1-3 cards with this before you're dead. It's just not enough to reverse a dominant board state. If an opponent opens with more mana than you, and quickly establishes better threats (bigger Ballistas, more active Ravagers because they drew more artifacts, etc.), your draws will be better, and given enough time, you'll swing the game your way, but you're still only able to play one land per turn, and Coercive Portal really reads something like "draw a card, take X damage, where X is the amount of power that your opponent put on the board instead of playing Coercive Portal".

Coercive Portal is a card that has utility in a Shop deck that is dedicated to control. If this card had been printed back when we first broke out Espresso Stax in January 2010, I'm pretty sure we would have found a way to work them in. If it had been printed in the early days of Terra Nova, there may have been a case for them there, then, too. As long as Shop Control is dead (and it's pretty dead), I just don't think this card is good enough. If we get any of our toys back (which I can't imagine happening any time soon, as the community at large seems to be happy those cards are restricted), maybe a Shop Control deck can be built, and maybe this card has a home there.

@rat3de

Like @Prospero said, that kind of effect to be good in controlling Shops/Stax lists. But I've found Bottled Cloister to be much better in those lists as it pairs so nicely with Ensnaring Bridge. The controlling lists are regrettably not Tier 1 by any stretch, but they can be just competitive enough to be fun. Of course, if you're looking for an effect to keep in against blue pilots, then Cloister is a big liability given what can happen when it is stolen or destroyed. This is not an issue against Shops, of course, but still can matter against Dredge on the anti-hate plan. In any case, the Cloister/Bridge synergy still did wonders for me against Shops/Dredge with my controlling lists this year.