I have the setup you are after on my MTB, I have used it racing, all day epics, multi-day bikebacking touring and day to day commuting.
I have tried a few different chargers, but fur ruggedness, mall size and value for money, I would recommend the Kemo 172, you need to cut and connect the wires into the Exposure cable, but well worth the fuss. The Exposure Boost cable will keep a Garmin charged, but would struggle with a smart phone, plus I recommend using the power port to connect to the long cable redeye rear light, so you have a dynamo powered, sper bright, rear light too.
There’s some more info here:
FORUM DISCUSSION WITH MY SETUP & AIDAN HARDING’S IN (both MTB setups, for racing, not the normal touring setup)
BLOG ENTRY SHOWING MY SETUP (with similar, but no longer available, Softhema charger)
My Revo light with standlight (after a few minutes to walk the bike across the beach)
Also, here is a pic of Phil Simcock’s setup for the Highland Trail 400, Exposure Revo + Kemo 172 to charge the Garmin in the day:
CLICK HERE
Hope that helps!