Can I hand wash 100% rayon and just lay flat to dry?

You would be taking a big chance. It could shrink a lot.

I’ve done it many times, in cold water. I’ve never had an item shrink, but sometimes they come out super wrinkly, and ironing rayon is not advisable.

Yes @augustlan very wrinkly too.
I once hand washed in cold water, a dress I had worn only once. It ended up at least two sizes smaller and a lot shorter. :-(
I have washed some 100% rayon shirts and not had them shrink, just get wrinkly.

I won’t buy rayon for the above reasons any longer. I’ve had very bad luck with Rayon shrinking and wrinkling and I’m too thrifty to spend money on dry cleaning.

Don’t do it. Wrinkle is right. 100% Rayon sucks.