We have those in CA too. Here you can actually tuck it behind the review mirror (as long as it still "transponds") so it doesn't look too bad.
I think that if you mounted it flat on the dash it would work though. I've loaned my transponder to people and used it in rental cars where I just layed it flat on the dash and it worked. The one thing you have to be careful of is the cops. In CA, the man hides out by the transponder readers and checks not only for speeders, but people without them (here there is a seperate, non express lane for people who pay cash) running through without paying tolls.
I could be wrong, but in CA I believe not properly displaying your transponder is a ticketable offense.
Hope this helps