Claudia Hinz observed a strange ‘rainbow’ on 5 May 2005 from Mt. Wendelstein (alt. 1835m). During a strong ice pellet shower from collapsing cumulonimbus there appeared an almost 20° long faint, but coloured piece of bow. It was rather diffuse, ragged and very broad. The sketch shows the bow. The sun was 50° high and the bow was approx 8° below the horizon.
From meteorological experience, a dying cumulonimbus can only produce ice when it collapses from the top and in the lower levels the temperature is low enough that the ice does not melt. That was the case here, the cloud collapsed and decayed fully into graupel. There was definitely no rain, which I had also not experienced previously with a ice pellet showers.
I do not want to commit myself whether there can be an ice pellet bow. However, the ice grains may occasionally be covered with a water layer, giving a smooth outer surface. Of course, the refraction in the ice-water interface must also be considered. If that interface was uneven it could explain some of the bow width.
On 11th of January 2006 Christian Fenn reported crystal-clear ice balls (photo 1, 2), in sleet falling at an air temperature of -4°C. He searched for an ice bow formed from the headlights of his car. In this picture the possible ice bow is compared to a (terrestrial) rainbow. There are also images by Christan Fenn of the ice balls.