Phase abbreviations for non-aqueous solutions
According to IUPAC, general notation for any solution would be $\ce{X(sln)}$ [1, p. 54]:

(vi) States of aggregation
The following one-, two- or three-letter symbols are used to represent the states of aggregation of chemical species [...] The letters are appended to the formula symbol in parentheses, and should be printed in Roman (upright) type without a full stop (period).
$$
\begin{array}{ll}
&amp;\ldots\\
&amp;\text{s} &amp;\text{solid}\\
&amp;\text{sln} &amp;\text{solution}\\
&amp;\text{vit} &amp;\text{vitreous substance}\\
&amp;\ldots\\
\end{array}
$$

Similarly, when denoting relation of a given quantity/symbol, subscript $\mathrm{sln}$ notation is used, e.g. $Δ_\mathrm{sln}G^\circ(T,p^\circ)$ [2, pp. 241, 276]:

4.2 Subscripts and superscripts
4.2.1 Subscripts
$$
\begin{array}{ll}
&amp;\ldots\\
&amp;\text{r} &amp;\text{reference, reduced}\\
&amp;\text{sln} &amp;\text{solution}\\
&amp;\text{s} &amp;\text{saline solution}\\
&amp;\ldots\\
\end{array}
$$

References

IUPAC “Green Book” Quantities, Units, and Symbols in Physical Chemistry, 3rd ed.; Cohen, R. E., Mills, I., Eds.; IUPAC Recommendations; RSC Pub: Cambridge, UK, 2007. ISBN 978-0-85404-433-7. (PDF)
Gamsjäger, H.; Lorimer, J. W.; Scharlin, P.; Shaw, D. G. Glossary of Terms Related to Solubility (IUPAC Recommendations 2008). Pure and Applied Chemistry 2008, 80 (2), 233–276. DOI: 10.1351/pac200880020233. (Free Access)