While I only have first-hand experience on few airliners, I have never heard of an alert announcing the passing of the transition altitude. In my opinion, such a system would be plagued with one major problem: the transition altitude is not at a fixed value all over the world but is subject to national airspace structure and also the terrain situation around the field. For example, in VIE it is at 5000ft while a few miles further east, Hungary uses 10.000ft. IIRC, Innsbruck has 10.500ft and nearby Salzburg uses 4000ft. So I think it is sufficient to have the trans. alt noted on the charts for instant reference - these are on the yoke anyway when departing/arriving. Same with the Transition Level, which takes the QNH and OAT into account on top of the Trans. Alt. and is delivered by ATC usually either via ATIS or in a descent clearance. Now, setting the Trans. Alt in the Altitude Alert System would lead to two things: firstly, the airplane would level off at the selected alt., which it is not supposed to do most of the time. Secondly and most importantly, this leaves the probability for a level bust at the actually cleared Alt/FL. Resetting the selected Altitude to where ATC wants You to go after transition sounds rather error-prone. In case it might be forgotten to switch altimeters right at the correct altitude/FL, this action is linked to the Climb and Approach checks in my company to provide a subtle reminder and this system seems to work just fine.