I researched and compared travel insurances that include cover for backcountry ski touring for 28 days in Canada for a NZ-resident couple in their thirties. Most travel insurers exclude skiing outside of the ski area, or off-piste. These are the options I found. Most also offer annual options which are comparable to 4-week single trips.

DISCLAIMER: I am not qualified to give financial advice.

Tugo, via Alpine Club of Canada

UPDATE For a small claim, Tugo Insurance took 9 months of processing and achieved nothing. I recommend avoiding this company.

• Premium: CAD $141.96 for emergency medical only, with default deductible (excess) of $150 and medical sum insured (a limit) of $50,000
• Destinations: Canada, some short transit stops
• Policy: http://www.alpineclubofcanada.ca/wp-content/uploads/2015/06/TuGoPolicyWo...

Best price by a large margin, but it excludes non-medical cover. The medical part covers accident, injury, sickness, evacuation, rescue, medical fees, hospitalisation, follow ups, repatriation, and accompaniment. This makes it a good insurance to supplement any existing travel insurance to get cover for backcountry touring.

Bupa / IHI

• Premium: USD $384
• Destinations: All but polar regions.
• Policy

Bupa seems to have a pretty comprehensive policy. I read the whole document. It also covers professionals, including athletes and guides. Apparently Nitro Circus athletes have this insurance!

New Zealand Alpine Club

• Destinations: All but polar regions (case by case)
• Premium: NZD $1264, including one $110 membership, which makes both adults eligible and includes some other benefits and discounts.
• Pricing
• Policy

Underwritten by Allianz.

ACE

• Destinations: All but Cuba
• Premium: NZD $296.97
• Policy

I investigated this one least thoroughly. I recommend calling to ask and confirm how they differentiate "mountaineering" from backcountry touring. You should also read the whole policy document (as with any insurance). I only searched it.