Have you looked at Thermacare? http://www.disboards.com/showthread.php?t=1657481

As far as I know, the neither the FAA nor the TSA specifically prohibit their use but I'd think it could potentially be a fire hazard. Hot plates (which use similar mechanics as a heating pad to create heat) are definitely not allowed under the FAR's - and that goes for crew use in the galleys as well as passengers.

OP, do you know off chance what equipment your flights will be using? Especially when it comes to the 737 fleet, in-seat power availability isn't consistent:

• 319: None
• 320: EmPower (F)
• 735: None
• 73G: AC Power (F/E+)
• 738: AC Power (F/E+)
• 739: AC Power (All, F/E+, or None - check the seatmap and watch for substitutions)
• 744: AC Power (F and C)
• 752 domestic: None (with the exception of one bird that does have power in F/E+)
• 752 p.s.: AC Power (All)
• 752 international: AC Power (All)
• 753: AC Power (F/E+ - except on birds used on TPAC flights)
• 762: EmPower (J and the first few rows of Y)
• 763 domestic: EmPower (F)
• 763 two class international: AC Power (All)
• 763 three class international: AC Power (F and C)
• 764 refurbished: AC Power (All)
• 764 unrefurbished and "H" config: EmPower (J and the first few rows of Y)
• 772 domestic: None
• 772 two class international: AC Power (All)
• 772 three class international: AC Power (All)
• 772 three class Hawaii: EmPower (F and C)
• 788: AC Power (All)

Some (primarily pmCO) aircraft have outlets for coach passengers (sometimes only in E+), some only have it in F, and others don't have it at all. Also, don't forget that even if .bomb says your flight has power right now there could always be an equipment swap close to departure. It happened to me a few months ago, we went from 739 with AC power and DirecTV to an unconverted 738 that didn't have power, IFE of any kind (not even overhead monitors), or Economy Plus inside T-24.