I was interested in doing the same thing.
I noticed that gootspoor had some videos on their site with automatic uncoupling:
Go to
http://www.grootspoormovie.com/
select 'video's' and then 'loc's'.
look at the films for LGB2044, LGB21423 and LGB28004.
I asked Michael at Grootspoor how it was done.
He said you can get a special coupler, which can be attached to a servo. I guess you wouldn't be interested in this but he also told me that some DCC decoders, such as the latest Massoth ones, can control the servo.
He didn't say much about the coupler and who made it. I think it may be this one by ZIMO:
http://grootspoor.com/catalog/product_info.php?cPath=28_1493&products_id=28545
I never got as far as actually doing it. I'll keep my eye on this thread.
Good luck.