Basically transformation can be done vice versa by just swapping the signs. So if your MGI to WGS84 Parameters look like this:
...the corresponding WGS84 to MGI Parameters would be:
BTW: Keep in mind that the schema of these 7 parameters is:
There are some specific details in each software which could make this not fully correct. In ArcGIS for example you also have to be aware of which transformation method is used, because there are different variants of this Helmert (7 Parameter) Transformation. Depending on which you choose, you have to or dont have to swap the signs of the 3 turn parameters.
Unfortunately I am not experienced enough to tell you the exact way to perform this transformation in QGIS or with proj.4, but I think when you know, that the parameters are the same, only signs have to be swapped, you should be able to do it.