So! I cut open one of my supposedly crossed squashes this afternoon. I am more convinced than ever that it is a cross between a pepo and an argyrosperma. The skin was very hard - it took my largest knife and a fair bit of pressure to get it open. Seeds look plentiful, plump and lovely. Flesh was pale, and extremely bland when tasted raw. It took a looooong time to cook; 1 1/2 hours at 400°F. and the texture was still quite firm, almost crunchy; the flavour was still very mild but actually quite pleasant.
I can find little information about pepo-argyrosperma crosses, although I did find a comment that they were "rare". It also appears, the fine apparent quality of the seeds notwithstanding, that they are likely to be infertile. I have popped 8 of them into a damp coffee filter in a baggie by the fireplace; I'll see what, if anything, happens.
I took a picture of the insides, and also I want back and looked at a photo taken of the squash bed in the summer. In it, you can clearly see that the plant from which these squash came has the typical argyrosperma silvery markings, even though it was saved from a White Lebanese (pepo) zucchini.
Edit: no, I'm wrong about the leaves. That's actually a pepo (zucchini) - I was thinking I was looking at this bed from 180° different spot. The leaves from this one are in fact very large and shallowly lobed, dark green and fairly low, and behind the second post. This was taken in mid July and they had not yet taken over the entire bed as they had by the end of the season.