I used iMacros before, too. I remember what you mean. You could build the macro inside the scripting language and then send it to iMacros for execution without saving it to the hard-drive.
UI Vision can not do directly the same, but almost. If you want, you can still build and create the macro inside your script. The only difference is that you need to save the JSON macro file before calling it. That is only one line more 
The UI Vision macro follows standard JSON syntax, that makes it easy to create the macro file programmatically. Then save the macro to the hard-drive into the UI Vision macro folder (or sub folder). Then call this macro via command line and make sure you use &storage=xfile
So if your file is called macro.json then the command line to start it is
"C:\Program Files\Mozilla Firefox\firefox.exe" "file:///D:/test/ui.vision.html?macro=macro.json&direct=1&storage=xfile
See also