Not sure how good the trails would be for skiing. A local would know better but the last time I was on a trailed hike in the catskills, conditions would not of been good for XC skiing there.
But, the Phoenicia-East branch trail from Denning to the Slide Mtn parking lot is about 10 miles round trip. People ski down the main trail from Slide but that's more of an AT style trip and you mentioned rolliing hills.
If by the backside of Belleayre Mtn, you mention from Lost Clove, that is an option, there I think are actual ski trails from the XC ski area at Belleayre but you could go in from Lost Clove Rd (off of Rt 47, south of Big Indian) and that would be a decent climb to Belleayre that gets steep right at the leanto. (skins might be useful). Once you get to the false summit of Belleayre (where the tower used to be) you could venture to the top of the ski lifts or take the left to head towards Balsam. I think there are a couple of chutes just past on the trail to Balsam you might have to take the skis off but once the trail levels it would be a nice ski to get to Balsam. (The trail is very faint and overgrown at some points, this isn't a very popular trail). Once you get towards Balsam though there is a steep climb you might have to take the skis off again.
Another option I see is perhaps skiing to Overlook from Prediger Road, via the Devil's Path to Echo Lake to Overlook.
There are also some good ski trails in the SW catskills by Mongaup Pond that I'd love to check out. I don't know the ski conditions on the trails. The last time I was on a trail was doing Lone/Rocky on Dec 26th and the conditions then were awful, icy and just packed. They should have a little more snow now because they've had some moderate snow recently, but it's still over thick ice. The last hike I did was Friday/Balsam Cap but that was a bushwack.
Another option I think could be going in from Biscuit brook off of Slide Mtn Rd (47) and going north towards Big Indian/Fir. You'd have to cross the brook itself by the leanto but that is a nice ski and is not overly popular.
I'd be prepared to carry skis in spots. This weekends snow storm is in fact going to hit us here down in southern NY/NJ more than the catskills, they say 3 inches in Kingston and up to 6 in my neck of the woods, which means I'll probably be skiing down here.
P.S. This should be in the Q&A section since it has to do with the NE.
Jay