I recently started working for a company that sends handwritten letters (yes, written by real people) on behalf of clients. We're letterfriend.com. You can find us and our competitors if you Google "handwritten letter service".
By far the most common use of mass (handwritten) direct mail is in the area of real estate. If you're looking to do small batches you could probably do wedding invites, as we get occasional requests for those. Google Adwords would allow you to target those people.
If you want any other ideas for niches, ask. I can take a look at some of our smaller requests.
FYI, real estate people (prospectors especially) are all about ROI. They will usually not be happy to pay $4 per card, even it is handwritten. People are paying around $1 for a printed letter. Super fancy wedding invites may be different.
Us, and our main competitors all have web apps that allow people to send small batches of letters.