IMO, for OpenRefine's user communities, the priority isn't 'trainer support' but rather reducing the need for extensive in-person training; this would allow trainers and users alike to spend more time actually using the tool doing their actual jobs.
I'm indeed co-ordinating the train-the-trainer program for Wikimedia OpenRefine users. I'm already noticing that time investment for myself and for the course participants is much higher than originally planned and budgeted. At least for the Wikimedia movement, which relies on volunteer work, on usually small and very tightly funded affiliate organizations, on donor money, and which works with usually quite underresourced partners, any cut in necessary training time and money would be extremely welcome.
Although OpenRefine has excellent documentation and tutorials available, my experience is that many/most users barely engage with them, even when pointed at them with great emphasis. And folks who have actually checked online tutorials and documentation still need help, sometimes over the long term. As software goes, OpenRefine requires an unusually high amount of human time and effort in training and mentoring. To alleviate this, it would be an enormous win if OpenRefine would make its interface more intuitive and conducive to self-paced learning. Some time ago @lozanaross brought up the idea of an in-app guided tutorial (similar to what you see in many video games); that would be amazing to have (and in first responses to a current survey for Wikimedia users I see that there is a lot of support for this idea). Removing jargon and generally making OpenRefine's UX more intuitive to use would also be so helpful.
I notice this not just with trainers, but with all users. The forum and other places where OpenRefine is discussed are very tech-oriented and perceived as intimidating. Asking trainers (who usually do training as a small side gig besides a day job) to report back to OpenRefine's community is a very big ask. In that regard, the best type of support would actually fully alleviate trainers' work; the most helpful thing OpenRefine could do for trainers (and users in general) would be to create a helpdesk-like separate infrastructure (manned by a helpdesk person / product manager or community liaison type) which interfaces with all users.