I think I have stopped the weep past the transmission shaft at the sliding disks.
My method:
Separate the sliding disks, and invert the inner disk. This puts the felt washer pocket towards the clutch basket.
Inside that pocket, I JB Welded a shaft grease seal (McMaster Carr #5154T61, or 1 1/8" ID x 1 3/8" OD). The seal lip just barely closes over the transmission shaft, when the inner primary cover is fitted (Atlas anyway).
Adjusting the primary chain requires some care: since the seal is pliable, when the gearbox is adjusted, the transmission shaft will push the seal and sliding disks off center. The primary chain should be adjusted over-tight, about 1/4" up/down play, then the gearbox moved 5 flats on the adjuster nut towards the front (loosening the chain). The 5 flat turn of the adjuster bolt should re-center the disk and seal. Vibration will help perfectly center the disk and seal.
The felt washer is compressed by flipping the inner disk over. This also helps prevent oil migration.
On my first run after making this mod, I found a bit of oil weep at the transmission shaft, but this stopped, possibly because the oil weeping was residual from before the mod. or possibly vibration better centered the seal. So far about a 100 miles and no weep.
Slick