ThunderbirdPortable command line arguments?

Before I installed TBPortable.exe, on my PC .eml files were associated with the standard TB.exe. When I switched to the portable version I changed the association to launch the TB.exe file inside the TBPortable\App folder. However, that association didn't work well, because it launched TB.exe outside of its portable environment, so TB ended up running with an incomplete profile. So I reassociated .eml files with TBPortable.exe. That fixed the incomplete profile issue but introduced a new problem. When TB.exe is already running, double-clicking an .eml file doesn't open it, because TBPortable.exe doesn't seem to pass the .eml file argument to the running instance of TB.exe. Is this a know limitation of the launcher, and could it be fixed? Thanks