Maybe not exactly what you want, but you can mount /tmp as tmpfs in memory:

--8<---------------cut here---------------start------------->8---
(file-systems (cons* ...
                         (file-system
                           (mount-point "/tmp")
                           (device "none")
                           (type "tmpfs")
                           (check? #f))
                         %base-file-systems))
--8<---------------cut here---------------end--------------->8---

-- 
Pierre Neidhardt
https://ambrevar.xyz/