Nope, Biscuit brook trailhead is north of Frost Valley YMCA and not in their property. There is a very porous fence between the YMCA and doubletop though.
A good loop would be to start from Seager and bushwack to Graham (may be thick in some spots towards the summit) and then over to Doubletop. Then bushwack following the state park boundry more or less (plane wreck and boundary markers lead you to the south peak) back towards the trail from Biscuit Brook, then you can hit Fir and then bushwack towards Big Indian (beware of the drainage) then continue north to hit the Shandankan trail past the leanto back to Seager.
Would be a long day but certainly doable from both ends. But from Seager, it kind of avoids have to go over Doubletop twice after going over to Graham if you started at Biscuit Brook.
If you do decide to do this over two days, you have some good options for camping and you can do Balsam Lake mtn (firetower) also... But it means you'll be bushwacking with a backpack. (not a big deal except for possible thick stuff on Graham and Doubletop). Big indian and Fir are open enough not to really need a small pack.
If you do this over two days, when you are by Graham, consider taking the old access trail down towards Balsam Lake and you can stay at the lean to on the other side of Balsam Lake fire tower on the trail towards Balsam Lake itself. There is also a leanto at Shandankan creek you can stay in or probably a better location is the leanto at Biscuit Brook itself.
An interestesting 3 day two night trip, allowing for driving time is perhaps to drive to Mill brook road and simply hike Balsam Lake the first day, stay at the leanto. Second day, hike back over Balsam Lake, go do Graham, then go to Doubletop, then over to Biscuit Brook leanto. If you still have time and energy, you can slackpack to Fir to save time on the third day. Then on the third day, you can do Fir (if not before), otherwise, hike up the biscuit brook trail, tag Big Indian (really quick bushwack!) and then out on the Shandakan trail. You would then have to walk the road back to Mill Brook road though but it's a pleasant walk in the country side. (Mill brook road though will be ALL uphill!)
Jay
Jay