Any decent quality chromatic tuner can be used, but I'd narrow the range down to two. Sabine makes one of them; I think the model number is AX-12. It doesn't require a jack since it just sits on top of the sound board (or any flat area) & picks up the vibrations.
The other is the one "ddw" mentioned; the "IntelliTouch" from Onboard Research. Best tuner I've ever used: very sensitive (even with the lowest strings of my acoustic bass guitar), with a large accurate display. Costs about the same as the Sabine or Korg's top models. One caveat 'though: the IT has a swivel joint that doesn't look too durable - be gentle with it.

Rick - might you describe your 'harp for the good folks? The way you have it modified seems to me to be the most versatile setup I've seen.