I do not belive one can do that through python. The Merge command on the Editor toolbar isn't accessible.
You would need a complex workaround; something like
0. identify the polygons you want to merge
1. create a layer with the identified polygons (the ones you want to merge)
2. run Dissolve_management on the layer (this makes a new output FC with the dissolved polygon)
3. delete the features in the layer you created in step 1 (which removes them from the FC)
4. append the output FC (with the dissolved polygon) to your original FC
Rinse, repeat
Are you sure a dissolve of the whole FC on one or more fields wouldn't do what you want?