I managed to work around the issue by using perl to compute the port number instead of bash. For some reason any permutation of bash -c 'echo <expr>' resulted in unwanted single quotes in the final expression and for some reason this does not happen when using perl -le 'print <expr>'
Here is the final expression I ended up using:
exeflags=`--bind-to \`hostname -i\`:\`perl -le 'print 6000+(10*$ENV{"LSB_JOBID"}+$ENV{"LSB_JOBINDEX"})%1000' \`