Personally I would not recommend using the roll type of loft insulation. It can be made up of strands of fibre glass (although you can get mineral fibres). Even putting it in a plastic bag may still lead to some fibres being left on the outside of the bag. Glass fibre strands and honey are not a good combination. There may well be no apparant contact with the insulation, but at a later date you don't know if there will be(eg using a feeder).
For the sake of a few pounds I would think it better to use a less risky option.
I have worked in a food and medicines factory for 13 years so may be over cautious.
Actually Rab, I've recently united several colonies and now only have 7, which is 1 or 2 more than I usually overwinter and I've had no problems in previous years. I'm not discouraging the use of insulation in general, just the use of loft insulation. As I mentioned, I've previously used carpet offcuts and old bath towels (over my first couple of winters), but haven't used anything for the last few.I've had no losses through the cold!
It's easy to say that when we have over ten hives...