There's nothing wrong with “plot magic”, and in fact, as a 2e DM you're severely handicapped by restricting yourself from using plot magic because the system is built with the firm knowledge that DMs know they can use so-called plot magic. (The cultural revulsion against GM fiat and anything that even looks kind of like GM fiat was born during the 3e era, which is why 2e doesn't have the explicit rules about what the DM is and isn't allowed to do that people now expect as part of a D&D edition.)
To make the methods available to your players, simply move the locus of the plot magic until it's not really “plot magic” at all. Instead of plot magicking up the pocket dimension of this adversary, just fiat up the spells and rituals the adversary used to create it, put those into the world, and let them be learnable by the PCs through discovery or research just like any other custom spell or ritual. You already have the game's permission to do this kind of thing.
Balance concerns are understandable, but somewhat misplaced. Official 2e material isn't balanced in the sense that is usually meant in the post-3e community, since strong balance wasn't a design concern of the system – every home table had its own view on whether balance mattered, and something that was balanced in one group would be unbalanced in another, so no official attempt at balance-as-we-know-it was ever bothered with. By making your own custom spells, you're actually more likely to create something that is balanced for your table than if you used homebrewed spells developed by someone else for their own group.