I think you can reach out to the front desk to request it. They are happy to accommodate guests.

It really depends on the policy of the different onsen hotels. My suggestion is that you do not choose those old and traditional ones (traditional ryokans). They do have a traditional to uphold to. The newer ones are more open-minded, and they are more prepared to receive foreign visitors who may have tattoo. They in fact would provide you with the stickers if you just tell them when you check in. Some hotels even state in their websites about the tattoo matters. Hope this helps.

Meat can be changed to fish. Meals are served at the restaurant.