That's pretty much the solution, though there are variations on the theme. For instance, you could store the document in DropBox or another cloud service, and import it each time you need it. That keeps you from accidentally overwriting the original.
You can also design the letterhead entirely in Pages. While the iCloud version of Pages is kind of sparse on features, the iOS version is more powerful. It will also sync to the iCloud version, so that you can see it on your Windows computer.
I was going to provide you with a link to an older thread where I talk about templates in iWorks, but I can't find it. But I do have the text in Evernote, so I'll paste it below. It's a bit out of dare, but still 'basically' sound.
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What’s a Pseudo-template?
In this case, it’s a document you’ve created and placed so that you can easily make copies of it with little to no danger of overwriting the original. What you lose over a real template is the ability to set placeholder images, text, etc. Editing the document will be exactly like editing the original, except you’ll be editing a copy.
Using iTunes Sharing: No iTunes Required
After you create the original document, with the fonts and/or other defaults you want, tap the Share icon, Send Copy, then iTunes. Use the Pages format to save it. This places a copy of the document in a special area reserved for coping files to and from a computer using iTunes. It will stay there unless you intentionally delete it either on the iPad, or in iTunes when the iPad is connected.
The next time you want a document with those defaults choose the Copy from iTunes option instead of Create Document. It takes the same number of steps as Create Document, so it's not a big hassle once you've set it up.
The advantage of doing it this way is that you never have to worry about accidentally editing your original, because a new copy is created, just as if you had used one of the templates.
If you want to delete a document copied to iTunes on the iPad, start out by choosing the Copy from iTunes option when creating a new document. Once you see the list of documents you can swipe them to get a Delete button.
I'm not sure how stable the iTunes file sharing buffer is. It may be possible to delete it through some sort of reset (though I've never seen it happen). That means you should also have a copy of your templates saved elsewhere, preferably not on the iPad, just like any other important document.
Using WebDAV: Internet Required
Another, similar solution is to use a cloud file service that supports WebDAV. Box.com does this, and I use it this way fairly often. The steps are exactly the same, except you use the WebDAV menu items instead of iTunes. The first time you choose the WebDAV option you will have to set up the server. Instructions should be on that service's site, somewhere.
The advantage over the iTunes method is that you are also creating a backup copy of your templates. One that won't get deleted if something happens to your iPad. It's also a good way to save your important documents, ones you can't afford to lose. You can't rely on iCloud for this, because iCloud is mostly meant to sync your documents between devices; it updates much too fast to be considered a backup. Mistakes become irretrievable very quickly.
The disadvantage of using Box is that your templates are online and can not be used without an internet connection.
Other:
You could also use a local file storage app like GoodReader, or use local (favorited) files in DropBox, Box, Google Drive or other cloud services. However, with the recent iWorks file format change this has become tricky. I don’t recommend it until Apple (and the others) figure out how to handle the format reliably.
Addendum: I'm pretty sure this last part has been corrected, and that most iOS app handle the iWorks formats correctly now.
