Quick one as promised to give heads up if the marriage is actually possible. Anyway, both of the components are not compatible. Officially. What I’m describing below is (successful) attempt to save my ass, rather than 100% planned action. So if you want to follow me do it on your own risk, and be careful so you can send the rotors back and have a full refund.
The Shimano rotors are stiff and light (due to steel-alloy sandwich technology and thick alloy spider). In my opinion the price (£21) is reasonable for the quality.
In my case the alloy spider was the only problem. Shimano have completely different calliper system, where pads are fit from the top, and they’re combined with bigger plates and heatsinks (quite neat system, just wondering how effective).
as you see below Avid got completely different system:
They have small “handles” to make the pads installation easier. So basically after fitting the rotors I realised why they won’t fit. At first glance is visible that the only problem is right pad. The small “handle” is overlapping about 2mm the spider arms. Scratched my head, pull out hacksaw and I’ve cut it. I deliberately left about 4mm to leave something I can grip on with pliers in the future.
And voilà! Slightly forced but marriage is possible ;-). The spider is just missing the calliper about a 1mm or so, phew! Quite lucky escape, but judging from scratches on the rotor, in worst case an extra washer (added underneath the calliper) will do the job in case of too small clearance.
The only problem might be removing the right pad with fingers but you can easily remove them with pliers.
Basically, handlebar change (to road one) forced me to install mechanical brakes I’ve chosen Avid BB7. Then I didn’t want to buy Shimano centerlock to 6 bolts adaptors (SM-RTDA10) but also didn’t want to pay 40 quid for one Avid rotor. So I’m aware that I created the whole situation by myself ;-). Anyway strange exercise, but with happy ending.