Use WSLENV to append to an environment variable

It would be nice to be able to use WSLENV to append to environment variables rather than replace them.

My main motivation is running a Windows executable in a Linux shell. I can do this now but I have to make sure my PATH contains all the Windows paths that are needed before I do a

export WSLENV=PATH/lw

I'm mainy adding directories to the PATH containing DLLs needed by the executable I am running. I would much rather set my LD_LIBRARY_PATH, which is the natural thing for Linux, in my shell script and then get WSLENV to append it to the Windows PATH, so maybe something like

export WSLENV=PATH+LD_LIBRARY_PATH/lw

This would mean append the Linux LD_LIBRARY_PATH environment variable to the existing Windows PATH environment variable and convert the paths in the LD_LIBRARY_PATH while doing it. The Linux PATH environment variable would not be exported.
So if my LD_LIBRARY_PATH was /mnt/c/Users/davidhunter2/bin then C:\Users\davidhunter22\bin would be appended to the Windows PATH whatever that happened to already be. Obviously with a ';' separator if needed.

Start by reproducing the WSLENV=PATH/lw example and review how WSLENV currently handles environment-variable replacement and path conversion. The change would need to define and implement append semantics for a Linux variable such as LD_LIBRARY_PATH, including Windows PATH separators and preserving the existing Windows value.