Does drush mung backticks and vertical bars? If not, you could use
cd `ls | grep foo- | head -n 1`
If backticks don't work, but |, $, ( and ) do, then you could change the above to
cd $(ls | grep foo- | head -n 1)
If | doen't work, but $, ( and ) do, then you could do
cd $(myprog)
where myprog is a script that you write to determine the directory name.
Also -- I don't understand how you might be able to use find to help you do a cd,
but, can you end your -exec with a +?