It would work but, only under certain circumstances might this be legal. It is not recommended. It could even be considered illeagle by the NEC (National Electrical Code.)
#12 Romex (Romex is a trade name of wire that a lot of people associate with the type of wire you are referring to. Like Sheetrock is a trade name of drywall or gypson board.) has a rating of only 20 amps at 250 volts, it is not rated for the temperatures associated with PV arrays, it is not outdoor rated, and it is not rated to be "strapped" directly to metal, and it cannot be used inside of a conduit from box to box, although it can leave a box in conduit and exit the other end of that conduit into wood framing.
If it were to leave the PV modules J-box in conduit and penetrate the roof where it could then exit the conduit and be stapled to wood, it might pass an inspection. There is still the amperage and temperature ratings to consider.
(In my opinion, the temperatures at the PV module (167 degrees F) are not that different to the temperatures in an attic space (140 degrees F.) This is where your local electrical inspector would have to make the final decision.
Here is a link to acceptable wire for PV array source circuits.
http://photovoltaics.sandia.gov/docs/John_Wiles_Code_Corner.htmNote that the NEC is revised every three years. The lastest is the 2008 Code.
http://www.nfpa.org/catalog/product.asp?pid=7008SB&order_src=A291Also available at your local electrical supply house.
If I might, make a suggestion? Make friends with an electrician? They are not that bad of people once you get to know them. Some electrical contractors can be real prima donna's though, but then, can't we all from time to time?
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