I have a solution to this
my old way was to use a tool
name: mp3 gain
path : "c:\program files\audio stuff\mp3gain.exe"
parameters "/r /c "%_path%%file"
for all selected sources.
as expected this launched a seperate comand window for each file . this was ok for 10 or so files but for larger groups it was bad.
so I changed to use an export config
this creates a batch file with a line for each selected file
and then the program asks if I want to view it. when you say yes it opesn the batch file and runs it.
it woks fine for up to about 1000 files.
I tried it last night on 6000 files because I wanted to Regainthem and it crapped out when trying to export the tags ( to create the batch file.)
I am sure tha the same thing could be done with wsh instead of bat. if you were good with wsh (vb, js) you could do some greats stuff.