Question: or the reverse, where a repeating expansion in a variable base to be rational?
been trying some trial and error cases without success

by variable base we mean each digit can be in a different base but any digit can occur as many times in the same base or not

Just in case anybody tries to pull a fast one no base $\sqrt 2$, $\pi$ , $e$ etc. can not have the irrational number itself as base, that would make this trivial.

Answer: 
Consider $.1141144114441144441144444\dots$, where the 1s are in base 3, and the 4s are in base 9. The expansion is nonrepeating, but it represents $${1\over3}+{1\over9}+{4\over9^2}+{1\over3\cdot9^2}+{1\over9^3}+{4\over9^4}+{4\over9^5}+\cdots$$ which is $${4\over9}+{4\over9^2}+{4\over9^3}+{4\over9^4}+{4\over9^5}+\cdots={1\over2}$$ which is rational.
Using bases $\,3,3^2,3^3,3^4,\ldots\,$ with corresponding digits $\,1,2,3,4,\ldots\;$ yields

$${\large{\sum\_{n=1}^\infty \frac{n}{3^n}}} = \frac{3}{4}$$