Support for Custom Toolchain

I'm very in fond of PIO's features and its integration into VS Code. However, there is one requirement that prevents me from using it actively for work, and that is the need for custom, vendor-specific, toolchain (compiler, linker, assembler, etc.). I am using a MDK-ARM (paid) toolchain. I have already tried solving this unofficial way through inquiry on PIO Community, here. But many errors are keep popping up, too many things might get hard-coded somewhere and is fairly difficult to identify and change, etc. Long story short, I'm unable to integrate a specific compiler support in PIO this way.

This specific toolchain probably cannot be implemented as open-source package (like other PIO packages). It's free version is available only for use with Arm Cortex-M0 and Arm Cortex-M0+ cores. Maybe it would be nice to have an option of providing files for a specific toolchain by yourself. This way you could use a package that might not be publicly available but individuals with specific reqirements could use it.

The issue names no files, tests, or entry points. Start by locating PlatformIO's toolchain and package-resolution code, then determine how a user-provided vendor toolchain could be configured and validated without requiring a public package; done should include documented support for the proposed workflow.