I see the case of Utawareru_mono as most similar to Ookamikakushi and Sora_no_Woto. Among the examples listed by Soljashy, all are written with appropriate kanji except for Yume_Miru_Kusuri, which is a single katakana phrase.
Utawarerumono and Ookamikakushi are written as a single hiragana phrase while Soranowoto is written as five individual katakana syllables separated by dots. What ties them all together is that they are purposefully not written in kanji and should not be interpreted as if they were.
It is neither consistent nor helpful to apply grammar rules to phrases where the creators specifically removed the grammar. This is easiest to see with Ookamikakushi, where the title is a portmanteau of Ookami (wolf) and Kamikakushi (spirited away). It is not possible to portmanteau the phrase using kanji and luckily the title has been left as a single unbroken phrase on danbooru.
In the case of Yumemirukusuri, the game was licensed and released in English as 'Yume Miru Kusuri: A Drug That Makes You Dream'. Breaking the title into 3 parts helps to make it readable and adding a translation of the title as a subtitle is effective. This is a good translation. However, tags on danbooru are not translations. Tags should keep what grammar is in the original and add no interpretation of their own.
For this reason I am in favor of using Utawarerumono, Soranowoto, and Yumemirukusuri.
The example of Mononoke_Hime from forum #33382 is also good. Mononoke is written as an unbroken hiragana phrase as opposed to in kanji (物の怪) and thus we keep it unbroken in the tag. The 姫 (hime) in the latter portion of the title is written in kanji and so we place a break at that point.
Ultimately, the alias system makes these arguments a mostly moot point. As long as all common usages are identified and aliased the end result is similar enough that I won't be upset by whatever decision is reached.