I can hardly believe what I'm reading here......
I have two datalogging multimeters. The reason I have two is that the first one I bought has crappy software. It turns out that the second has crappy software too. They were bought to replace a dongle that actually has good software but is unreliable. I was only wondering under the shower a couple of nights ago, that I might be able to put this stuff to better use by way of an Arduino.
It looks like this is possible.
Karma to you.
The meter to I have to hand is a UT61C. It cost me a lot more than $30 but looks much the same as yours. It has two opto eyes under a sliding cover.
Are you essentially just using a 10k in series to connect the meter to a digital pin? I have just pulled the D9 plug apart. There is a lot of wiring there but it doesn't mean it is all used at the other end.