For which $R$ is $\{0^a10^b10^c\mid R(a,b,c)\}$ context-free?
What you're looking for is an old result of Ginsburg and Spanier actually related to one of the oldest open questions of the field.   See Ginsburg's book The Mathematical Theory of CFLs.

Defs. A linear set is a set of the form $\vec{v} + P^*$ where $\vec{v} \in \mathbb{N}^k$ and $P$ is a finite set of such vectors ($P^*$ denotes all linear combinations of the vectors in $P$).  A linear set is stratified if :


All the vectors in $P$ have at most two nonzero components;
For all $\vec{x}, \vec{y} \in P$, the nonzero positions of $\vec{x}$ and $\vec{y}$ are not interleaved (this is related to the fact that $a^ib^ic^jd^j$ is CFL but not $a^ib^jc^id^j$).


Theorem. Let $w_1, \ldots, w_k$ be some words.  Any CFL $L \subseteq w_1^*\cdots w_k^*$ can be written as $L = \{w_1^{i_1}\cdots w_k^{i_k} \mid (i_1, \ldots, i_k) \in R\}$ where $R$ is an union of stratified sets.  Somewhat conversely, any language of that form with $R$ a union of stratified sets  is a CFL.

Naturally, there are languages that are CFL that can be written in the previous way with $R$ being quite arbitrary—e.g., with $w_1=w_2=a$ and $R = \{(2^i, j)\}.$ 
 However, if for instance all $w_i$'s are different letters, then $R$ has to be a union of stratified sets for $L$ to be a CFL.

As for the open question I mentioned before, the following problem is not known to be decidable:


Given: A finite union of linear sets
Question: Is this set presentable as a finite union of stratified sets?