Thanks for the thoughts.. the trouble with 1/4 inch dowels is that the frame is mainly 1/4" timber, so the dowels would be guaranteed to blow out the sides.

After a bit of rudimentary testing, I went with the 3mm bamboo skewers. The glue seems to stick to bamboo okay, and I solved the moisture for the polyurethane glue issue by dunking the dowels before putting glue on them and jamming them in the holes. I found that if I stopped part way through the insertion, that was it. It had to be done in one motion otherwise the dowels would bind in the holes never to move again.

The frame feels more rigid now which can only be a good thing.

I'll post some pics once the frame is done.