Sorry it's taken so long to respond to your question - I went away on holiday and it's taken me a while to get back into normal life again!
I think that a tenant cashback scheme is, in principle a good idea, but it needs to be well managed. My direct experience of working on the ground is that some residents are incredibly knowledgeable about repairs - to the extent that many of them are frustrated that they cannot (aren't permitted) to undertake their own repairs, as they feel they could do a better job than a contractor.
I suppose the biggest risk, especially to larger providers of social housing, is that by allowing residents to undertake their own repairs, economies of scale may have to be sacrificed to permit greater choice. Efficiency savings have typically been one of the greatest drivers behind the centralisation of maintenance services (and the need to control standards centrally) so this is quite a big consideration.
On the positive side, allowing those that want to take control of their own repairs to do so would surely increase their satisfaction levels, providing the overall shift did not increase their rent (through an increased need for a local inspection regime - although if this could also be organised through co-production that might offset any increase in management costs).