Greetings, everyone!
I think that, ultimately, the choice of emergency metal ingestion and burning is limited primarily by two factors which, so far as I can tell, have not been fully clarified (kindly do correct me if there are WoB on this that I missed):
1). Is the metal burning process limited specifically to the stomach, or it can take place anywhere in the digestive tract? Or, indeed, as long as the metal is located within the allomancer, it can be burned?
2). Need the metal surface be in contact with the lining of the stomach/flesh of the allomancer?
For part 1, most likely we can eliminate the latter possibility, since burning allomantic metals anywhere in the body of the allomancer can be lethal - e.g. one does not wish to burn up all the iron in the body. If one assumes burning of allomantic metals is limited to stomach only, then most logical oral-based metal ingestion methods have been outlined above. However, if we assume that one can burn the metal anywhere in the digestive tract (e.g. Zane might have kept the coin in his mouth not only as an emergency landing pad, but also as an extra source of metal - the lad was creative), then there is another possibility - instead of focusing on the mouth, we can consider the other end of the digestive tract. Advantages include: unexpected place, well concealed from sight; villains, especially in that time period, are unlikely to check there, it will also take them precious seconds to get to the metal "compartment" at which point the allomancer can burn it; in addition, it is very easy and, dare I say, natural, to expel unused metals.
For part 2, I vaguely recollect a WoB on how it is the structure of the metal/alloy that acts as a catalyst/key for enabling the allomancer to use a specific ability. Thus, if the ability is fully dependent on the metal lattice structure being in the allomancer, it could be that concealing the metal in something else, undigestible, is a possibility - then, as proposed before, one can put metals in capsules and swallow at leisure. I think there is one obvious example which might indicate that this is possible: people swallow aluminium (e.g. Vin in the original trilogy) and it works - yet under normal conditions, the surface layer of aluminium is covered by aluminium oxide, which lends rise to a different atomic arrangement on the surface and prevents the lining to be directly in touch with aluminium metal per se, yet the metal nugget remains allomantically active.