Roland (and Korg and Ketron, and even Yamaha occasionally) actually DO alter their source code from time to time. They are called UPGRADES..!
Now yes, you aren't ever going to get a customized OS just for you. But if what you want is something that generally hugely improves the product, there's a good chance that it might be incorporated into the next OS release. Thing is, a) you've got to let them know your suggestions (post at Roland-arranger.com, send emails to technical support, try sending directly to the R&D division if you can), and b) you've got to try to work out IF what you want done can be, especially in mind of the fact that closed OS's can only get up to a certain size, only address so much memory, etc., etc..
Dom at Lionstracs is probably BY FAR the most responsive developer likely to incorporate a good feature quickly into the OS, followed probably by Ketron, but Y, K & R, not so much. You get what you get, when they give it to you...
But don't let that stop you from at least TRYING.