Sani to Afriski via Katse Dam.

I do not have any first-hand experience of motor-biking through Lesotho and don't own a bike either. However, when entering Lesotho via car I have never had the license disc checked at the border. This definitely does take place at police check points along the roads however, and this tends to be on the main, tarred rods. I have been through police check points several times just outside Butha Buthe, and chances are you will go through it too on your way out. I would advise you to get that disc replaced. Its one thing getting caught without one in SA, but getting caught without one in a foreign country, especially in Africa, could land you up in more hot water than its worth taking a chance for.

You do pay a road tax on entering Lesotho, but as far as insurance goes I would start by checking with your insurance company what their coverage is for your bike in neighbouring African countries.

We did once intend connecting Ha Lejone at the northern end of Katse with the A1 near Letseng. We did not land up doing it at the time, but we managed to find out that the road is drivable but pretty rough going as far as driving in a car goes (ie need high clearance and probably 4x4 if muddy etc). I would guess that on a bike it would be manageable too. Assuming you are getting to Katse from Sani along the main roads, I cannot imagine you will have difficulties on a bike, since these are all drivable by vehicle. Snow can be an issue though, especially on the passes.