This question came up locally today:
Your discs are soaked and there is no drying them. Can you put a rag between your hand and the disc to throw?

801.03 Artificial Devices
A. During a round, a player shall not use any artificial device that may directly assist in making a throw, except those devices that reduce or control abrasion to the skin (such as gloves, tape, bandages, or gauze) and medical items (such as knee or ankle braces). [...]

If the player is using it like a glove, I'd allow it. But if the player is throwing the towel along with the disc, I would not put it in the same category as a glove anymore and disallow it.

Might be interesting to develop a glove for throwing in the rain made of material that works well with wet discs.