How to configure VSCode to compile Typescript using dynamic paths to locate modules?

Question

I have a complex, legacy JS project that I would like explore converting to TS. The first major hurdle is the fact that modules are resolved in a kind of quirky way: specifically I want tsc to look for them inside the project directory as normal, but if they aren't there, tsc should then look in the location of a ruby gem that is in use (I know, its odd). In other words, module paths need to be determined at run time.

Building the project has been solved. What I need to do next is find some way to show VSCode how to find those modules. Any specific ideas on how to go about this? I've been exploring making a language service plugin but have not been able to find a clear way forward.