I managed to have the spacebar act as the "pause" button of WinAmp by modifying the accelerator tables in the executable with Resource Hacker (http://www.angusj.com/resourcehacker/). For instance, I edited the accelerator with ID 40046 (which was
VK_C, corresponding to the current shortcut for the Pause button) in the three accelerator tables containing it (1371, 1372, 1373), by replacing it with
VK_SPACE (then you have to click the Compile script button on the top).
In one of the accelerator tables (1372) there is already a VK_SPACE accelerator. I just removed it (as leaving it there was preventing the spacebar to act as Pause/Unpause if the playlist window was focused). Not sure what the space was doing before in the playlist.
All the other default keyboard shortcuts of Winamp could be changed the same way.
Note that you will probably not be able to edit Winamp.exe in-place in its installation folder (except if running Resource Hacker with elevated privileges), so the easiest is to make a copy somewhere else and copy it back. Make sure anyway to keep a copy of the original executable, in case something goes wrong.