Have you taken a look at AOPA's renter's insurance? I haven't looked
at all the details, but it looks like it meets your requirements. I
looked at the FAQ's and it specifically lists coverage of non-powered
sailpanes, including experimental. You are required to get injury and
property damage coverage. The minimum is $250,000. As an AOPA member,
the annual premium is $81. Non member is $86. The optional non-owned
aircraft damage coverage with a $20,000 limit premium is $283. Non
member is $250. That totals to $364 per year for a member. You can
check their web site at http://www.aopaia.com/renters_start.cfm#.

Charlie