Assuming there is enough room around the outside of thge cap screw's head a very viable option might be the external type bolt extactors. You can pound one into place over the fasteners head and then use a ratchet and extension to turn it off. Using a penetrant spray and maybe even some heat on the threaded section will usually encourage things breaking free.

Here's the tool as its available from Sears but there are similar tools made by Irwin and Hanson that are available at Auto Zone and the like. Good luck!

http://www.sears.com/shc/s/ProductDisplay?catalogId=12605&storeId=10153&krypto=kZBasrgDQsd3cyMgUUDejR0NgxHZ4yUuFggMAeqcaaZS7WGQQqqiIyVlZDWUfGsj4GAO6v73I8lL%0A8NfdJRZfxPR%2B DTEPwvIT0PaXy81OX9%2FjXjW%2Fy2kDzr%2BaN2qOdcKotRQ6VKQZuCGFu7IKyu34WxeU%0AGcE8tk4q91qFa90MM8PL6vHX7EB Lfw%3D%3D