Yes. Triggering any action in Girder from a webpage is easy. There are two files (girder.js and ajax_sendevent.hmtl) that are part of the default Girder install located in the httpd directory.
Just add: <script src="girder.js" type="text/javascript"></script> in the <head> tag of your html and to trigger any event add: onclick="return SendEvent("THUMBSUP", "100"" inside the html tag you want to trigger an event. (You will have to change THUMBSUP and 100 to the event and device number you have setup in Girder as your event.)
This is a basic building block. There are SEVERAL variations on how this can be done, so if you have something in your head let me know...
Is the Event identified by the GML and the ASCII string that it is setup to send; [my.gml\onkyo\keys\POWER ON\!1PWR01]
Where I am using the Serial Pluggin with asimple lua to set the appropriate parameters, is the device number the Pluggin number (242 for Generic Serial))?
I've been using Girder with IR; using serial and the Webserver is a new world for me.
You can right click on an action and choose add event. Once added change the device and event string to match what you have on the webpage and it should fire.
I think your page should be working. I'm away from my Girder machine, so I can't test right now. Add a script action in Girder with the folllowing: gir.TriggerEvent( "ON", "242", 0). Run the script action, if it works then there is a problem with your webpage, if it doesn't there is a problem with the event name and/or device number.
Again, I am away from Girder, but do see your statement: OnClick="return SendEvent("RCVR OFF", "242");" has nested double quotes... should be something likeOnClick='return SendEvent("RCVR OFF", "242");'