The bigger risk is not freezing (if they have some charge) but actually irreversible sulphating.
Over here (Canada, gets nippy in winter) I tell customers they have two choices for their lead-acid batteries in cottages over the winter (assuming they don't want to haul them out). If there are solar panels that will occasionally see sun (i.e. at an angle steep enough to shed snow) then disconnect everything from those batteries but the charge controller. That will keep them charged and happy for as long as it takes (5 months here).
If the panels are likely snowed in for the winter, then I advise that they make sure their batteries have a full 100% charge. That part is important: It means going through a full absorb cycle, possible even doing an equalize to make sure they're topped up completely. Then disconnect everything. They'll sit like that for 5 months and bounce back in spring with a few charge cycles. Many have followed this and so far it has worked very well.
There's something else that helps as well; the low temperatures in winter slow down the chemistry (of self-discharging and sulphating). Making it easier for batteries to sit for long period without damage.
If there is a choice I do suggest to people to put their batteries inside, so they follow the average temperatures (of -20's Centigrade) rather than the nightly extremes (of -30's Centigrade or below).
By the way, hydrogen gas is a non-issue. Batteries only gas when they are being charged and the charging Voltage exceeds a certain threshold. On top of that, even if you were to store them inside and let them gas while charging you'd pretty much never reach an explosive mixture of oxygen-hydrogen. It's near impossible to contain hydrogen to that extend, it's a very small molecule that rises and escapes through the most minute openings (of which there are plenty in the average house). I've never heard of a hydrogen explosion due to a battery bank, other than in the battery itself (blowing the top off, we've had a few of those). Of course, if you have a battery bank indoors while in use, I do very much suggest to put them in a vented battery box! The smell alone while charging will convince you that's a good idea!
There's an oddity we found with indoor battery banks and hydrogen: Your average carbon-monoxide alarm is much more sensitive to the small amounts of hydrogen escaping from the batteries (even if they are in a vented battery box) than they are to carbon-monoxide! For very air-tight homes we've had to direct-vent the battery box and gasket the lid to keep the CO-detector from going off.
-RoB-
