1. Is your target customer a software developer? It seems like your customer would be a executive, perhaps at a smaller business. You want to get that person's attention. I think open-sourcing is more likely to get developer attention.
2. There are lots of other marketing strategies you could try (content-based marketing, going to tradeshows, cold calling, etc).
3. If you have existing customers, I think you really need to work with them on in-depth case studies that explain why they benefited from choosing your solution. You should also see if you can market to other, similar customers.
4. "customizable by any developer" - That developer should be you. This I have personal experience with. We initially delivered a product and left it to the customer to configure. Later, we discovered that customizing the product for the customer delivered better results, better relationships, and allowed us to learn about our own product and our customers.
Anyway, I would do a lot of sales/marketing brainstorming before I open sourced. It seems you see it as a marketing strategy, but I think you should consider other avenues first. Good luck!