/comclose closes the connection to the COM object, but the file which exports that COM object progid remains locked. I presume that mIRC has locked the file, although it might be MSDTC... however closing mIRC unclocks the file. The big problem here is that mIRC has to be closed every time I need to recompile, and it would also mean that for each upgrade version that is released, users would have to close mIRC to upgrade.
/comclose means that mIRC is finished with this COM object, so there is no reason to lock the source file. If it really is mIRC's fault, then this should definitely be rectified.