Why not this way:
Let say a BAS module is a only outsourced text parts of the main module.
The users can divide a too long code into different text snipplet. Each shown in a separate TAB.
Or the use can outsource text-parts, which already contain 100% error free code.
Now the compiler could re-import them into one big main module before start the compiling. In this moment they all are one file.
This needs no change in the compiler. Classes would not be touched by this. Ok, you would have to do a lot in adjusting your Auto-Syntax-Control.
I know this from my old basic dialect
#Import Standard.bas
#Import Record.bas
#Import ConstDef.bas
#Import LoadResources.bas
...
This would also give us the chance to close TABs with code we do not edit at the moment, but of course should stay in the project.