Is this just a specific case of the limitation that "This plugin doesn't work within titles, links and other substitution patterns, for technical reasons."
Yes, the sqlquery plugin does not allow other syntax plugins to be called for the part between it's own tags (
<sql>...<sql>). But if you are only interested in getting this single case working - use var-plugin-syntax in sqlquery-plugin-syntax - you might want to try to place a feature request at the sqlquery plugin issue tracker (see
https://github.com/digitalduke/SQLquery/issues). There is no guaranty that it is going to be changed but it's worth trying.