Why refrigerate food when you can just stamp spoiled food so that it was stored properly before?
You'll have to ask Brandon how that'd interact with gastric acid breaking down the stamp. Or how porous/loose material interacts with stamps in the first place.
Come to think of it... There's a WoB saying the Nightwatcher could change your species, but have a hard time making a spren bond to you. So... could the Nightwatcher turn you Scadrian and make you eligible for Allomantic powers? Or does the Nightwatcher's boons operate on soulstamp principles?
Hell, let's say you bought a vial of the wrong metal on your field trip to Sel. Could you pay a Forger to stamp the vial into being a vial of the right metal (it's believable that you would check before such an important trip) and then drink the metal contained in the vial to fuel your Allomancy?
All right, all right. Let's see... /u/Aurora_Fatalis, changing metals around with other forms of Investiture is generally going to work, according to how I view the magic right now. The power is there, you just need to align the matter the right way. So forging new metals: not too difficult. This is because Allomancy isn't actually using Investiture in the metals, but using it as a key to get power from somewhere else.
Forging a sword to be a Shardblade, however, would be very, very difficult for multiple reasons. The most obvious one is that the Investiture required would be enormous. A Shardblade is a highly-Invested object, with its own self-aware soul.
If you could overcome the initial resistance invested objects have to being influenced by other magics (something that Forgery is particularly good at doing anyway) you'd theoretically be able to change Shardblade/spren's personality like you could a person's.
Fooling the magic via Connection and Identity is not so hard, under the right circumstances, so making a Forger into an Elantrian (or an Allomancer) for a short time is plausible. Making yourself into a Radiant, however, would be more difficult--because the limitations placed on that magic have to do with persuading a sapient being you are worth the bond.
How about regular food? If I stamp a pineapple pizza into a pepperoni pizza and eat it, what nutrients do I end up with?
The way I have it working now, I believe (though I'd have to do some double-checking, as it's been a while since I've been working on Sel) soulstamps are more fragile than things like Aons, and it would be very hard to eat something with one without breaking it. But assuming you could, you'd get nutrients from what it had become--but those would change back once the stamp broke or ran out.
It is possible to go so far down this rabbit hole, however, that the chemistry of Forging (like the physics of Allomancy) it just can't make sense any more. So be aware.
With things like Stamping metals for Allomancy, you have said that it'd be possible for short time, but then burning it would break the Seal and metal would revert back.
I guess it would be similar with food, right?
Yes, that's the big problem with Forging. Getting the stamp to stay in place once you start to change the object that has been stamped.