A free solution: Use BetterTouchTool to trigger an applescript similar to the following, which will activate Safari if it's running, and do nothing if it's not:
if application "Safari" is running then
tell application "Safari" to activate
A not free solution: Tie a BetterTouchTool trigger to a Keyboard Maestro macro, which will give you plenty of control over when the application gets activated. The following example would have the same effect as the above AppleScript:
There are many ways to tie BTT to KM, but the easiest is having BTT trigger the hotkey used by KM (in this case, CTL-OPT-SHIFT-CMD-S).
Obviously, both solutions would work for any application, not just Safari.