So I was reading Tamamura Takeji's 1978 anthology of Five Mountain literature (Nihon no Zen goroku 8: Gozan shisō 日本の禅語録八：五山詩僧) when I came across this one by Sesson Yūbai 雪村友梅:

函谷関西放逐僧
黄皮痩裏骨稜層
有時宴座幽巌石
只欠空生作朋友

A monk driven west of Hángǔ pass,
Wasted down to yellow skin and sticking-out bones.
At times I sit atop the rocks in dark and hidden places,
But there I lack even the friendship of 空生.

So the issue is how to translate 空生, kūshō in Japanese, literally "emptiness + create/life". The Iwanami Dictionary of Buddhism has no entry for it, and neither does the Nihon Kokugo Daijiten. Charles Muller's Digital Dictionary of Buddhism has it as an alternate title for Subhūti, and Subhūti was known as "解空第一", "Emptiness understander number one," so that would make sense. But Tamamura has a more exciting proposal: maybe it's Shunnyata-shin 舜若多神! This literally means "God of Emptiness" and appears in the Canon as a personification of sūnyatā, notably in the Śūraṅgama Sūtra:

T0945_.19.0123b29: ... 阿難汝豈不知。今此會中阿那律
T0945_.19.0123c01: 陀無目而見。跋難陀龍無耳而聽。殑伽神
T0945_.19.0123c02: 女非鼻聞香。驕梵鉢提異舌知味。舜若多神
T0945_.19.0123c03: 無身有觸。如來光中映令暫現。既爲風質其
T0945_.19.0123c04: 體元無。...

"... Ananda, didn't you know? Among those gathered here are Aniruddha, who has no eyes but sees; the naga Upananda, who has no ears but hears; the Goddess of the Ganges, who is noseless but smells; Gavampati, who is odd-tongued but tastes; and the God of Emptiness 舜若多神, who has no form but feels. A temporary projection in the light of the Tathagata, it has the nature of wind but no original body...

Tamamura offers absolutely no justification for his reading, and it conflicts with other reliable (I assume) sources, like the Dictionary of Five Mountain Literature Terminology (Gozan bungaku yōgo jiten 五山文学用語辞典), which not only glosses 空生 as Subhūti but also mentions this particular poem as an example of such usage. So it probably isn't wise to take Tamamura's theory too seriously. I'm posting this mainly because I did not know that sūnyatā had been personified like that. Seems to undermine the metaphor a bit.