Following the paper of Giusto and Simpson (Located sets and reverse mathematics) the Mandelbrot set $M$ is located closed if the distance function $f:\mathbb C\rightarrow\mathbb R$, $f(x)=d(x,M)$, exists in the model under consideration, which I assume you take to be the model containing only computable objects. Such locatedness seems to be the same as Conjecture 4 of Hertling ("*Is the Mandelbrot set computable?*", Math. Logic Quarterly, 51(1):5-18, 2005), which asks whether $f:\mathbb C\rightarrow \mathbb R$ is computable. What happens if we replace $\mathbb C$ by $\mathbb Q[i]$? Could there be a way to compute $f(q)$ using a representation of $q$ as a rational, but nevertheless no way to approximate $f(x)$ given an arbitrary $x$, due to a lack of a useful modulus of continuity in $x\mapsto d(x,M)$? No, because $|d(x,M)-d(q,M)|\le d(q,x)$.