Just because you have something that will relay or forward (that device that has access to both L2s) doesn't mean its going to work with alexa or google home or homekit, etc. etc.
Not without some major background work and setup most likely, and understanding the details of how your device you want to say wakeup X actually does that..
My Alexa can turn on my TV, and off.. but I have my harmony remote in the same vlan as alexa, while my tv is in its own vlan. Both of these vlans are different than my other vlans. It would prob work without even... Since the harmony remote isn't in standby and the alexa should be able to talk to it over L3.
But trying to find ways to move L2 data into another L2 is not the right approach.. Correct design of your L2s is better option from a security standpoint.. You need X to talk to Y via layer 2 - then put them in the same layer 2, its really that simple!!! Isolate that network from your other stuff..
Do you trust alexa... do you trust your tv, do you trust your iot - well no that is why we isolate them.. But if X needs to talk to Y via layer 2 stuff.. The simple solution is just put them in the same L2 ;)