Well, first you'd have to be a feruchemist born in MaiPon on Sel, as only people born there, or with a bloodline connection to the area, can be forgers. Even so, this seems unlikely to me. We know that forging takes relatively little investiture, and is unable to effect other even lightly invested objects. So, let's assume the metalmind is empty. A soulstamp will only hold if the lie is plausible. You have to convince the object that this thing has happened to it. So, we will also assume that you've used this metal mind before, but it is now drained completely.
With all this, we finally get to the main issue: where does the extra investiture you want to put into the metalmind come from?
I could lie to a person and say that I poured water into their cup. I could even convince them that they were with me at the well earlier, and that's where I got the water. All of that breaks if the person looks into their cup, because it would still be empty.
I do love a good attempt at power hacking, though.