What percent of volcanoes are glaciated?
21.7%, by my calculations (338 / 1556 holocene-active volcanoes). I calculated perennial snow by combining 6 weeks of MOD10A2 data from winter and summer weeks in 2014 to figure out which pixels had snow both in the summer and the winter. My source code for that is available as a gist (an older version accidentally classified cloud as snow; that's been fixed). Then I checked if these pixels fell within 10km of the volcanoes in the Smithsonian Institution's Global Volcanism Database. I also plotted the woefully incomplete Global Land Ice Measurements from Space glacier polygons.

There are clearly a few false-positives on ocean islands... I think this is due to MOD10A2 marking choppy surf as snow. Looking into that. I intend to publish this and will put a reference here if it is published.

Here's what the map looks like for part of Alaska:



And the whole world: