The boinccmd line utility should give the control you are after, specifically
--project URL op project operation
op = reset | detach | update | suspend | resume | nomorework | allowmorework | detach_when_done | dont_detach_when_done
You could then suspend/resume tasks using for instance cron. Type
crontab -e
Assuming you have the command line version installed in the usual os x location, you can paste something like this (read up on cron for the syntax) to switch from worldcommunitygrid to rosetta at 12:
0 12 * * * cd /Library/Application\ Support/BOINC\ Data/ && boinccmd --project www.worldcommunitygrid.org suspend && boinccmd --project boinc.bakerlab.org/rosetta/ resume
Hopefully this should get you started