IIRC, not a chance of a full Siabod ascent but the lower half is a fun and up and down run. This can be combined with a play in the woods above Capel Curig on the non-Crafnant side. No route as such, but start from Siabod cafe, cross the road, up the lane and head in the direction (right) of plas y brenin. More a follow your nose run i’m afraid but there are tasty descents (and a pump track of sorts!). Do note though there is a freakishly large amount of walkers around at the moment (lots of rain and low-viz last weekend), so be accommodating. For a legal run that can be combined with the Marin, there’s always Crafnant itself that drops into Capel Curig. A lovely run down, but a pig of a climb that I’m told is totally cleanable.
Edit: sorry, just to clarify as people are asking for routes up to Siabod – head over the stone bridge from Capel (on the right hand side of the A5 as you head towards Betws and bear right (it’s signposted)).