Small batch cement mixers are great and have been used for a long time. But after doing some study, if manufacturing small cement mixers that can be efficient in people effort and $$ as well as environmentally friendly, making a small batch mixer similar to the one that Monolithic.com sells at their mixer sales page. Their mixer only works on skid loaders, but a version could be made to work on the OCE Tractor!
Another option that would be very interesting is to make peristaltic mixer, that does 'just in time' mixing. It puts just the 'right' amount of aggregate, cement, and water (and optionally various other additives). Currently there are mixing trucks that are available that do this, but if there is a commercial batch plant nearby they are typically not as viable as using plants and mix-only trucks.
One of the nice things about peristaltic mixing is it is mixed on site, when requested, in the amount requested, not in larger batches. It's limitations are that it needs the supplies available when starting the project on site, but this is normally what is done if making it on site in out of the way areas. The equipment also does not have to be as 'big' as commercial trucks. Mixing is done typically with an auger arrangements, so a small amount is continuously being mixed just before being delivered into forms.
Just a few thoughts. If not on a first version, possibly ideas for the future.