@Togger, in a case of emergency yes. Under normal conditions NO!
First of all, consider to taking your valve cover off again.
With a little bit of bad luck you got the wrong silicone and it bakes together...
Second, leftovers from your silicone might get loose and clog your lubrication system, with known result.
If you really have to improvise, take a very long o-ring material of fitting diameter, cut it to lenght. Clue it together with superclue, tackle it at your valve cover spotwise with superclue and put it in shape. Don't expect it to last forever, but as an on spot repair will do.
