It is undated, but has two comments, the earliest is dated April 2010, which has a link to a blog ... which 404s. Alas, both of the internet archive/wayback machine saves for the link also 404'ed back when they were first saved. But the blog still exists, so let's see if I can find the content.
Aha! May have been a simple date typo (dunno how that happened; possibly the author typed it in by hand? Redated the post? *shrug* She's got years of traffic driven to her blog's 404 page from a comment on Martha Stewart's site).
"I brought along strawberry hand pies from Martha."
So it looks like the Martha Stewart Strawberry Hand Pie show probably pre-dated the end of April, which makes sense, given the strawberry season. I have no idea how to find out which episode it originally aired in. And I don't care, because we have a lot of reason to believe at this point that Martha Stewart's crowd is the primary amplifier of the term. What did they intend to include in what is clearly a cross-cuisine category?
This is _savory_ hand pies, with a list of recipes that includes empanadas, pasties, pot pies, and a bunch of "hand pies" which have clearly have had their culture-of-origin terms erased entirely. (I wonder what that Alsatian potato pie started life as?)
The "hand pie" cross-cuisine term, however, predates 2010.
From 2009, an apple hand pie:
From 2008: a peach hand pie:
From 2007, a blueberry vegan hand pie:
They used margarine, altho they call it "vegan butter" in the recipe.
2006 Nola savory hand pies:
Martha Stewart claims this was from the October 2005 Martha Stewart Living; that might actually be checkable.
Oh, look here! An index!
Both the Alsatian Potato Hand Pie and the Cauliflower and Manchego Hand Pie appear in this issue. We may have ground zero for "hand pie" as a term.
Still here? Summary: Martha Stewart's crowd invented the "hand pie" as a cross-cultural term for savory and sweet pastries that can be held in the hand. It appeared originally in the October 2005 issue of Martha Stewart Living magazine. It was, from early on, if not the very beginning, intended to capture this idea from cultures around the world, including empanadas. And actually, empanadas survived on recipes within the Martha Stewart empire much better than terms from many other contributing cuisines.
I'm a little surprised it took 5 years for google trends to notice, and then a few more years for "hand pie" shops to start opening up in touristy places, to go with all the pinning on pinterest and so forth.
ETA: Crap. I posted too soon. I'm totally wrong on this. It is older. I'll be back later.
Okay. Still with Martha Stewart Living and back to the index:
So we're at least back to October 1996.
I'm not entirely certain how much I believe this (or the 1998) earlier example. "Hand pie" does not appear within the body of the recipe, only in the title, so it's possible somebody edited it; it would be nice to see the actual original magazine to confirm that "hand pie" really does appear within it. "Hand pie" occurs within the body of the recipes by 2005. It is not fruitful to search on hand pie and early dates places like epicurious, because a lot of older recipes are tagged "hand pie", but do not include the actual term (the term really did take off after 2010). This supplies further evidence that this is intended as a a culinary term that does two important things. The most important thing is it takes some weight off of the heavily overused and highly confusing "tart". The other thing it does is identify a particular kind of food prep (pastry shell, hand-sized, sweet or savoury filling that you took some care to reduce the sogginess of) that virtually every foodway has used sooner or later. Since the modern/current recipes are all going to do modern/current things with the fillings (our pasties are going to be comparatively lard-free, for example), there's little point in retaining the traditional terms. These are not the traditional items -- they are a contemporary item, deserving a contemporary, functional, identifier.