I've tried it now with various entity types, with and without blockbullets texture, with nodraw, various filters, etc. Seems to be not easily possible. It appears to me that the only satisfactory way to do it is by toggling the shields collision on and off. This, however, will result in the (portion of the) shield not blocking bullets as long as someone is standing inside it's wall.
You could create a new texture which blocks bullets but not the players, unles the blockbullets parameter blocks players as well.
I forgot to mention it. The blockbullets texture is basicly identical to nodraw, thus blocking everything apart from light. #Edit: SIGH! After messing a little more with the VMTs I've come to the conclusion that it's not possible. There is a "%compilepassbullets" 1 parameter, but there's no opposite (0 doesn't do a thing)
