The problem happens when you have a control already set using PlatformMP, and then try to change it using the Capture Key during runtime. Here's a really simple capx showing the problem.
http://db.tt/M6uldR3X (haha, the link says R3X
Oh, and if you use the Firefox error console, it comes up with a single error after the freeze:
"key_info is null
http://localhost:50000/Rex_PlatformMP_behavior.js Line 838"
The problem might still be with Keycode messing up while handing the number over to PlatformMP, but the source of the freeze itself is in PlatformMP. Hope that helps resolve this issue!