Hi Joe,
Unfortunately, there is no way via the API to query the Engine's cache to determine the sequence files that are loaded. You could possibly do this by adding a sequence file load/unload callback to each sequence file. Therefore when the sequence file is loaded you add it to your custom cache and when it is unloaded you remove it from the cache. There may be other ways of doing this, just thought this one would be the easiest to implement.
SG