When I was working with pension fund trustees at the TUC I was always a bit nervous about the idea of paying them. This was long before I got interested in the area of motivation theory, and was instead I think mainly a concern that payment might lead the "wrong" type of trustees to come forward and/or change the approach of those already elected.
As I've read more about motivation research, I would have thought that I would become more hardened in my opposition to paying trustees, but actually I find myself more open to the idea. I'm currently reading Motivation, Agency and Public Policy and it makes the case very well that financial rewards can work to 'crowd in' motivation when we're expecting people to act altruistically. This maybe because they are seen as a symbol of appreciation, rather than an incentive to increase supply...!
I think this may apply with respect to trustees, at least of big schemes. The job they have to do has become much more demanding both in terms of time involved and the intellectual effort required. If you accept Le Grand's argument, there's a threshold that volunteers or public servants need to cross in terms of self-sacrifice in order for it to feel worthwhile (counterintuitively too little sacrifice makes it seem less appealing). But there's also a threshold at which the sacrifice becomes too much. Perhaps the increased workload has pushed many trustees to the edge of the latter threshold, and some form of payment would at least be a recognition that their contribution is valued, and may make them stick at it.
When I think back to times I heard member trustees (who were TU members) arguing that they should be paid, it was generally argued in terms of the amount of work they had to put in. It felt like a request for recognition, rather than a demand for compensation.