A Gantt chart of designer hours?
An unusual request, but one approach would be to filter the Gantt view by resource type (designers -- hmmm...if you currenty ID resources by individual names, you might have to set up another resource-related field to ID them all as designers).
It's not a Gantt Chart, but there's also the "resource chart" view (or is it "resource graph?" -- I'm working on my Mac without MSP in front of me), which can be set for a range of time granularity and shows the use of resources for the project in question.
oooh...I just realized that you're talking about "different projects," which implies multiple files. If that's the case, neither of those approaches above will work. What you might try, but which I really can't vouch for, is to filter the individual project Gantt views for the resource type in question, then copy and paste the resulting tasks into a new file. Rinse, and repeat for the other projects, pasting them all into the same file.
It could work.
Good Luck.
