Yes and no, the implementation varies...
To support older browsers before Firefox 3, you may need to create your own NPN_PluginThreadAsyncCall function. On Windows, this can be achieved by creating a data structure capable of holding the function pointer and void* opaque pointer. Subsequently, post a custom message to the main window with a pointer to the data structure as LPARAM.
The main window's WINPROC operates on the UI thread, enabling communication with JavaScript. Upon receiving the message in your WINPROC, simply cast LPARAM back to the pointer, execute the method with the opaque data, and release the data structure.
For Mac, a similar approach can be taken using a queue to store events and checking for any during the NULL event (triggered by Mac OS roughly every tick). If an event is present, extract it, call the method, free it, and continue processing.
An analogous procedure likely exists for Linux, though specifics are currently unknown.
An example of the Windows version can be found in the firebreath project.
The winproc message handling is located in the following file:
https://github.com/firebreath/FireBreath/blob/master/src/PluginWindow/Win/PluginWindowWin.cpp
The event and data structure definitions are outlined in its header file:
https://github.com/firebreath/FireBreath/blob/master/src/PluginWindow/Win/PluginWindowWin.h
Lastly, the method for triggering that event is displayed here:
void ActiveXBrowserHost::ScheduleAsyncCall(void (*func)(void *), void *userData)
{
if (m_hWnd != NULL)
::PostMessage(m_hWnd, WM_ASYNCTHREADINVOKE, NULL,
(LPARAM)new FB::WINDOWS_ASYNC_EVENT(func, userData));
}