Here's the thing with solar hot water vs solar electric: I'm already going to have a WHOLE bunch of solar electric set up. All the solar panels match each other, use the same racking, need the same permitting, and I pretty much need to do the same work and meet the same requirements and regulations no matter how many or how few I put up.
Also, I can use solar photovoltaic YEAR ROUND to produce electricity. I can than use that electricity to do whatever I want - lighting, run a computer, charge an electric car, heat my house, cool my house, heat my water, run my washing machine, etc. etc. I can also get credited with my power company for any EXTRA electricity I produce during the summer and then get that energy BACK in the winter. I am NOT specifically creating electricity just to heat water. If I have "left-over" electricity, THEN pumping it into heat makes sense.
To add a solar hot water panel to the garage, it's a completely new set of building codes, regulations, and skill sets. Also, the water panel wouldn't match the PV panels, and due to size differences, I wouldn't be able to maximize my rooftop space. All that, and a solar hot water panel ONLY makes hot water. That's it! It does ONE thing. And when I would want that heat the MOST (in the winter, for heating) that's when the resource is the least available. Also, there's no simple/easy/cheap way to bank hot water over a range of 6 months.
Lastly, solar thermal is really all about surface area times how bright your sun is time number of hours it's shining. So, a 4x8' solar thermal collector could gain 32xHEATOFSUNxHoursofWinterDaylight. On the other hand, my garage also has two garage doors, each 7x9'. If I simply glaze in one of those, that gives me 63 square feet of solar collection for the same number of hours of daylight! And it does that without pumps or any other points of failure. (The vertical orientation of the garage doors is also a better angle for collecting the winter angle light than the slope of the roof.)
Don't get me wrong - Solar thermal and water based solar collectors are GREAT, they just aren't the right thing to add for space heating to a garage that's all set up for photovoltaic.
I'm also considering some sort of a wood-burning or pellet-burning stove that could be rigged up as a boiler. That makes a nice bio-mass heat source and pumping the heat into the slab is still a great way to temper the rate of heat. (It's easy for wood stoves running full tilt to make TOO much heat, but for too little time.) I need to check with the insurance company first to see how a wood stove in the garage would affect my insurance rates. Having a metal roof instead of a shingled asphalt one should help bring the cost down a little. Perhaps it would be a wash on insurance cost with a wood stove?
Lastly, I'm also thinking about the possibility of digging a deep trench to connect my house and the garage. I could then bury a pipe with insulated water pipes in it and run water to and from the garage. Theoretically, I could add a water jacket to the wood-stove in the house, and pump heated water from the house to the garage. I could then also tie in a solar hot water panel on the south end of the house (seriously, like the only other small patch of sun in my yard...) to the wood stove and the garage slab. All of that is possible, but would get complicated real fast too.