I usually use the last or the first image taken and outline copy and paste onto the final stack image. I have never tried it but possibly Registax can align the foreground while DSS aligns the backgraound. The 2 resulting images are then cut and pasted in a way to get a clear foreground and background.
http://www.waelect.com.au/mswhin63/W...2011_05_06.png
The one shown above link was a very rough cut and paste and you can easily see the outline. On my next ones I hope to find out how to define the outline better so as not to show the outline.