Sourcing and opening multiple files of same name, in specified folder, in Terminal? Is there a way?
It depends on whether fslview accepts several filenames at once and does the right thing based on that (the documentation/man page should have more details on this). If yes, you can run

cd folder
fslview data*/image_a


to have it open all files at once. If no, you can use

cd folder
for f in data*; do fslview "$f"/image_a; done


or

cd folder
for f in data*/image_a; do fslview "$f"; done


instead (the second option works better if there are data X directories which don't contain image_a).

PS: For a test run, replace fslview with echo fslview to see which command(s) would be executed.