I know a guy who has a restaurant in NY and he runs his truck off of used fryer oil. It can be done, but you need a reliable source of used oil and a garage to process it. Clean new cooking oil is US$6-$10/gallon in the US (compared to about $3.50 for gas from the pump) and the DR does not produce much cooking oil. Palm oil may have some potential, but I don't think it would be cost effective unless it was also used. Another potential source for biodiesel in the DR would be the pencil tree(which grows anywhere without inputs), but the technology hasn't been proven.
Corn as a source of biodiesel isn't cost effective and they are cutting the subsidies for it in the US. It takes something like 9 gallons of petroleum to make 7 gallons of corn based diesel fuel after fertilizer and processing and the corn industry is subsidized. There is some potential for other crops(such as switchgrass) that need fewer inputs that may work, but there aren't exactly any switchgrass farmer lobbyists in DC to get a program started. Monsanto/Cargill has deeper pockets. Canada isn't going to develop biofuels while they are still a major oil exporter.
There are other markets for used oil. It's used to make soap and sometimes plastic. As biodiesel catches on, it won't be free anymore, which is good for restaurants wanting to get rid of it. I wouldn't be surprised if some fast food chains start to use the oil to power on-site generators or even fryers. At the very least, they could use the oil for their fleet trucks.
Aside from car fuel, there is some potential for corn cob to be made into eco-friendly charcoal and there is some work going into making shrub trees into sustainable charcoal sources. Propane was subsidized to reduce the demand for charcoal and it's charcoal production that is the biggest threat to the island's ecosystems.