I don't know why more people don't do this - FX with acoustic. Sure, overdrive or distortion would be inappropriate (though I think John Martyn occasionally got up to such tricks) but you can have some real fun and get some beautiful effects.
Here's what I got with the accumulated GAS money...

A Line 6 MM4. I was torn between this and the Carl Martin Surf Trem. The Martin may have had the nicer tremolo effect but the Line 6
a) is not far off
b) does a lot more.
(but if you're looking for JUST tremolo, you should check the Carl Martin out)
After an hour or so's playing around at McGibbons today I am sold on tremolo and acoustic guitar. A lovely, big sound. It may be that this, and not the uni-vibe, is the sound I've been hearing in my head.
The line 6 has two tremolo effects. They name them "opto" and "bias" and say that "opto" is the Fender tremolo and "bias" is the Vox tremolo. I understand how the opto must work (photo cell and pulsed light) but don't have a clue about "bias".
Interestingly the "opto" way of doing things has a very distinct sound. You can hear the same thing in the uni-vibe (which also used lamps and photo cell).
So far I am opting for the "bias" tremolo. Less dramatic, but, I think, warmer. The MM4 allows you to modify the wave form from sine to totally square. I have "squared" the wave form just a little.
One thing I learned too late. You have to manage this kind of pedal carefully. I have loaded the 4 memories with settings that I like (my notepad reads
1) fast vibe
2) slow phase
3) trem
4) slow vibe
But I forgot to write the actual dial positions down. So if I ever lose what's in the memory....