I'm afraid a "translation" match is almost impossible for poetry. Poems are notorious for imagery and allegories. The possible permutations are almost endless.

Yosano's poems are rife with imageries and she uses classical (non-modern) styles alot.

After scanning almost every tanka she's written, my best guess is this one from midaregami:
夢にせめてせめてと思ひその神に小百合の露の歌ささやきぬ

I tried for 15 minutes with the Rexroth translation you provided but the book which the translation came from, (Women Poets of Japan by Kenneth Rexroth and Ikuto Atsumi,) did not quote the original source.

So what I did was use an alternative translation from Goldstein and Shinoda.

In dreams at least
I'll grant her wish---
And whispering to my love
Asleep beside me,
I recite her poem

Which led me to this article. (look at entry 80)

http://www.poemlife.com/TranslateColoum/fanjinghua/article.asp?vArticleId=31703

This article is in chinese so just in case you don't understand chinese I'll explain this.

The "Her" in the tanka refers to 小百合 or white lily ~ the nickname of yamakawa tomiko, another poet and tanka writer (which explains "her poem")

http://andrejkoymasky.com/liv/fam/bioy1/yamaka01.html

Yamakawa tomiko had something going on with Yosano Hiroshi (husband of Akiko). So the three of them became involved with each other ....

http://en.wikipedia.org/wiki/Tekkan_Yosano

Seems like a right interesting situation ....