Ok, so I decided to do a bit of tinkering and hack an off-the-shelf dimmer into my Finnex planted+ fixture. It works great so far but I just completed the task. I'll post updates later to confirm it keeps working for me.
In part I was inspired by this post:
And after some study and dissection/reassembly of the fixture, I came up with a plan.
Some warnings up front:
1) Do not attempt this unless you are good at soldering. In particular you should know a cold joint from a hot joint, and how to insulate connections properly with heat shrink, etc. While unlikely, if you do this horribly wrong the connections could short, heat up, and do bad things like catch fire. Don't burn your house down please.
2) I have professional experience in electronic circuits, soldering, etc. I'm not really qualified to judge how easy this is for an average hobbyist. For me, this was easy. I think an electronic hobbyist with reasonable soldering skills should also find this fairly easy. There's nothing terribly exotic going on here. Soldering the connectors is slightly tricky, as they tend to act as heatsinks, but if you crank the iron up too much they melt.
3) This WILL void your warranty. (btw Lowe, I consider my warranty void!)
4) This ONLY applies to Finnex lights using an external power brick that provides DC voltage under 24v and that draw less than 2 amps (I'd not trust cheap coax connectors above this without specs proving otherwise). In theory you can get decent quality coaxial power connectors that are rated to 5 amps, or even more, but I used cheap ones. The specific fixture I did was a Planted+ 30", which has a supply rated for 21w at 15v, which works out to 1.4 amps.
Some general notes on my choices:
I could have done a lot more elaborate design here, pairing a FET in with an arduino and done some nice programmable fade-in sunrise effects, etc. Some day I may expand this project and do something like that. However, I chose the simple approach here as my goal was really to get my light level down a bit. I also wanted to keep it cheap and simple as a proof-of-concept, I strongly believed it would work, but had no solid proof and a lot of mumblings on the forum of folks that were not sure it was possible.
So, what you need:
5.5mm x2.1mm male coaxial power connector, in-line type
5.5x2.1mm female coaxial power connector, bulkhead mount
4-amp 5-24v dimmer (I got mine at http://www.amazon.com/gp/product/B007RFMY1O/, but many similar products will work).
Opening up the Finnex (2 screws on the end), pulling the strain relief out with a pliers, and cutting the cord:
Hooking the power-brick end up to confirm which wire is positive vs negative. Note that on mine, one wire as a ribbed edge, the other is rounded. Make a note of which is which.
Soldering on the in-line connector (make sure to slide the jacket on first! Also, flux is your friend.) My dimmer wants the center-pin positive, I made sure to wire it that way (see above). Also, be sure to insulate these before putting the cover back on, otherwise it may press the wires together and short. (again, experienced solderers only please!)
After twisting the wires onto the bulkhead connector for the fixture, I did a test-run to make sure it works (I did not use the dimmer, just wanted to make sure the light would turn on):
The bulkhead connector, after soldering, insulating and screwing it into the hole that held the strain relief (near perfect fit!)
The fixture finished with the dimmer on high (pardon the camera lens flare):
and just barely on: