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