In reply to MooseMouse:
It's usually done as a day trip (not sure if you're planning on staying in the Grassen). A friend and I have looked at this even time we've been to Engelberg and it's on the to do list for 2014.
We've been told that it's 1x50m ab. And that it's definately 2x 25m abs. Both categorically... I've spoken to to guys who've done it with a guide with 1x50 though but I don't like the idea of lugging all that rope around with me.
Guides' offices tend to be open about conditions and the ski lodge is a good place to ask.
Last week we did Realp -> Albert-Heim Hutte -> Lochberg -> Goscheneralpsee and then down to Goschenen over two days. We'd hoped to keep going over a few days to Engelberg but my footbeds put paid to that
Snow's great at the moment:
https://www.facebook.com/media/set/?set=a.504383022942012.1073741825.102184...
The decent down to the valley isn't straight forward either: good vis and careful route finding are needed, according to the guide I spoke to. The 1:50k freeride map has a route marked roughly but you'll have an easier time following the path on the 1:25k