I roughly estimate that about half of all ryokan open their baths to non-staying guests. This means that there are usually quite a few baths to choose from.The following page has lists of baths that are open to non-staying travelers:
http://www2b.biglobe.ne.jp/~kondo/onsen/kaetsu.htm
There are 15 listings for Senami Onsen (g) and 20 listings for Tsukioka Onsen ()
For Yutagami, the page lists just one place, and it seems that a daytime visit by non-staying guests is coupled with a lunch:
http://www.oyanagi.co.jp/index.shtml