Acoustic Guitar Construction

See pic.
Are there dowels or something under those caps, extending into a bridge plate, meant to add insurance to the pinless bridge?

Either dowels or bolts. It is for assurance for a SS pinless. I have been told that it is not needed and does not help but it seems right to me. I did it on my acoustic bass. In mine it was not a cap but the top of a hard wood dowel.

So, your dowel went through the bridge, and the top, and all the way thru the bridge plate, right? I suppose it is not necessary, but it does seem like it should help, doesn't it?

Yes right through the bridge plate. I think it helps at least it made me feel better.

Logic would indicate that the fasteners or pins add strength or at least more glue surface. But -- a separation only occurs when the "glue" fails or shears away from the surface. So it seems the pins might "catch" the slipping bridge for a time but not necessarily aid in preventing the glue joint failure.

For a time Gibson used bolt on plastic bridges -- no glue.