I'd like to add my emphatic support for this feature. Now that I am using S3D to slice multimaterial prints for a Palette 2 Pro, the inability to swap out an updated STL file is wasting a maddening amount of my time. If after I have spent time prepping and perfecting a multimaterial print, I find out that one single STL component needs to be updated, I basically have to trash the entire construct and start over again.... Re-import STLs, align models origins, group models together, resize, reposition, open all four Processes, and re-assign them to each part of the model, rebuild all of your custom supports, and then re-slice. For a large, complex project, and for a troublesome STL that needs to be tweaked over and over again until it slices correctly, redoing all of these steps on every single iteration is absolutely painful.
Being able to leave a mostly completed multimaterial project as-is, and simply update a source STL would be a god-send feature... something I would absolutely pay (more) money for to be honest. And I agree with the previous poster...a manual process would be best, not automatic. I'm simply envisioning an "Update Model" button within the component properties page, that then lets you select the updated STL on your computer, and swaps it out in your project.
Please and thank you!!