Extra arguments dialog for custom actions

Custom actions are very useful to integrate external command line tools, like git-tfs, into SourceTree.

I would like to create a custom action for every command I need to use, but I can't because some actions need arguments that aren't provided by SourceTree or arguments that should be provided by the user, such as the Work Item number to be associated with the TFS check-in.

git-gui's (the default GUI for Git) custom action feature has an option to show a dialog with a text box and puts the user input into an $ARGS variable.

I've tried to start a PowerShell script from a custom action with no success (there seems to be some problem with quotes and paths containing spaces). The script would show a dialog with a text box and then call the actual command.

I think none of these solutions are ideal. Creating scripts for every command that need extra arguments is cumbersome. Showing a single input box and putting the value into a single variable makes the user deal with the command line options, which is exactly what I'm trying to avoid.

I wish I could specify the number of non-trivial arguments needed and a message to prompt the user for each argument. Variables could be named $1, ... , $N (the name doesn't matter).